Imagine the cars, the official cars. Long, slick, black, tail to tail as they slice through the traffic. Imagine the motorcycle outriders, the sirens, the bodyguards behind the smoky windows. Imagine the blank, resentful eyes of those who watch the cortege sweep by the old packed buses, the coughing, spluttering taxis, the shoppers with their fat bags full of fruit or fish or nuts, the pedestrians lined up waiting to cross, the beggars’ incoming alms from outstretched arms. The air of Shahid Bahonar Avenue full of exhaust and shouting and hawkers bellowing and revving engines.

Then a turn to the right, a short incline, a turn to the left. And peace. A calm tree-lined avenue. Neat soldiers with neat salutes, shiny boots and guns, polished accoutrements that reflect the cold, clear winter Tehran sunshine and the flawless blue sky that arcs overhead. The hum of tyres across smooth tarmac. The distant, dusty mountains. The leaves of the slender ash trees softly rustling and their shadows playing across the white walls of the Niavaran palace, and Princess Leila Pahlavi’s home. Later, of course, she would have nightmares. Running through the palace corridors, she knew she wasn’t meant to be there. The guards were chasing her, and if she stayed too long she would be found and they’d cut her head off. She didn’t want to stay too long. But no nightmares now. No nightmares for the Shah’s little girl.

She was born, the third child of the third wife of Reza Pahlavi II (the shah of shahs, the king of kings, the Light of the Aryans, the superior presence) in the military hospital in Tehran. Within hours, it had been renamed in her honour. Princess Leila Pahlavi was never going to have a normal life.

It was 27 March, 1970. The Shah had been on the throne for nearly 30 years. The Americans had invaded Cambodia. Opec was just starting its campaign to boost oil prices. The top-selling album was Simon and Garfunkel’s Bridge Over Troubled Water . Joe Frazier was heavyweight champion of the world. Jimi Hendrix was about to die and, in Britain, a state of emergency was soon to be called by the new prime minister, Edward Heath, as dock workers went on strike.

Iran itself was feverish with change. In the early 60s, the Shah, drawing on massively enhanced oil revenues, had launched a comprehensive drive to modernise his rural, religious, deeply conservative country. There were campaigns against illiteracy and for the emancipation of women; huge amounts of land were redistributed to the poor, health services were improved, and an extensive programme of industrialisation undertaken. But the social tensions generated were too much. By 1970, the pressures against his autocratic rule, with its conspicuous consumption and favoured elites, were growing. To mark the 250th anniversary of the ‘Peacock Throne’ a $60 million party, with 165 chefs flown in from Paris and a tonne of Imperial Caspian caviar, was thrown at Persepolis, the ancient seat of the Persian monarchs. The mullahs and the communists growled and muttered. The call to prayer howled out of the country’s packed cities and filthy villages full of bile and fury.

Not that the young princess ever heard it. She lived with her brothers and sisters in the palace in north Tehran. She had her rooms, her servants, her signed pictures of President Carter and his eight-year-old daughter on the walls. The Shah’s children’s private zoo was stocked with deer, antelope, monkeys, a lion and an Indian elephant given as a gift by Indira Gandhi. And she had her mother and father, to whom, everybody says again and again, she was extremely close.

Her young mother, Her Majesty the Empress Farah Diba, went to great lengths to ensure as ordinary a childhood as possible for her daughter. After the princess’s death, the Empress, who now splits her time between New York and Paris, wrote a letter to a British newspaper. It is addressed to her dead child. It reveals a lot more than it was meant to.

‘We made every effort to spend as much time with you as possible,’ she wrote. ‘But we could not be there always… this is the same with many working mothers. I could never bear it when your governess took you away from me.’

The letter is a thousand words long. It is curiously plaintive, almost apologetic.

‘At dinner, if you were all sitting quietly under the supervision of the governess, you would cause a riot the moment your father set foot in the room. Your brother would spit the spinach out of his mouth, your sister would throw up her napkins and leave the table, but you would reach for his arm.

‘For kindergarten, we organised a class within the palace grounds. Other children from different walks of life were selected to be with you, some of whom remained friends throughout your life. You were quite daring, a tomboy, and good at swimming, riding and cycling.’

I call Pari Abisaltai in Los Angeles. Researching an article about an Iranian exile means a lot of long-distance telephone calls. To Turkey, to Rome, to Paris, to New York, LA, Washington. One London-based Iranian – who had left Iran in 1979, as well – told me that the reality of diaspora is an address book full of very long telephone numbers. ‘If we were to be branded as exiles, the iron would be in the shape of an OO,’ he said.

Abisaltai is a journalist who became a close friend of the Shah and his wife in the last decade of their reign. She often accompanied the royal couple on their trips overseas, spent a lot of time in the palace and on Pahlavi family holidays. Again, she tells me how close the family were.

‘Leila always saw her parents in the morning,’ she told me. ‘Her mother often went up to visit her at lunch and the Shah would often walk over to pick her up from school. Her schoolmates all came from other good families who were friends with the Shah.’

So not exactly ‘from all walks of life’, then. ‘Yes, yes,’ Abisaltai says. ‘From all walks of life. The Shah and his wife had many, many friends.’ And she says, again, as everybody does, the Shah and his daughter were very close.

‘She could go into his office anytime she wanted. Even if he was with another president, she could go in and sit on his knee.’

Leila herself recalled the same. In an interview with a French journalist, she said: ‘Even when I was only three he would take me by the hand when he went to meet foreign dignitaries. Every day I’d go and find him in his office, even if he was in the middle of a meeting… Every morning he used to ask me to pray for rain for the benefit of our [dry]country’s farmers.’

In 1975, with unrest growing, the Shah banned the multiparty system. Dissent was fanned from France by the militant Muslim cleric Ayatollah Khomeini. The Shah’s reaction to the huge public demonstrations – unleashing the hated Savak secret service and ordering, or at least allowing, the army to shoot down hundreds in the streets – made things predictably worse. By January 1979, the country was in a state of virtual civil war. As the Empress put it in her letter: ‘the storm clouds were gathering’.

In 1979, the storm broke. After being rebuffed by a series of erstwhile allies, including Thatcher’s Britain, the Shah was by then gravely ill with leukemia and increasingly desperate for a safe haven. Leila was packed on to a plane with her grandmother and flown to a Texan airforce base where her eldest brother, Crown Prince Reza Pahlavi, was training to be a pilot. ‘We came out with nothing,’ the princess said later. ‘Thank God the governess brought the photo albums.’

A few days later, the Shah himself flew his personal Boeing 707 into exile to Egypt. Though the family had certainly left a lot behind, they were hardly penniless. No one actually knows the exact figures – probably not even the late Shah himself – but the Pahlavi offshore funds are estimated to have totalled more than $100m.

The next few months were a nightmare. The family was split up, reunited, split up again. All the time, the Shah became more and more sick, and the news was full of arrests, riots, demonstrations against the royal family in Iran or across the world. They went through the Bahamas, Mexico, and back to Egypt. The children stayed in America, and flew out when they could in order to see their parents. There were times when, for security reasons, the Shah and his wife had to leave a country in the middle of the night. Their children were left sleeping as the parents moved on.

‘She got up in the morning and she did not know where her mother and father had gone. That was a very bad experience for her,’ says Abisaltai. Leila, then 10, would hear reports of her father’s deteriorating health across crackling long-distance telephone lines.

‘Her mother would telephone every night and Leila would be crying and screaming. “Where are you? I want to see Daddy,”‘ another family friend says.

In Panama, during a brief reunion, Abisaltai says the young princess was so badly traumatised that if her parents left the room she would start howling in fear.

The family finally ended up back in Egypt as guests of President Sadat. The Shah died on 27 July 1980, aged 60. As the final days came, though the other children remained in Cairo with their father, Leila was sent to Alexandria. At the funeral, she wore a white dress. ‘She was so quiet, so tranquil, so unbothered by all the fuss and the commotion around her, it was an almost unreal calm,’ a close friend who was there remembers. But the hurt was there and was very deep. ‘The last memory I have of him is the most painful,’ the princess told a French magazine in 1999. ‘When I understood the end was near, they wouldn’t let me into his room. For a long time, I had the feeling I had betrayed him by my absence.’ In another interview she talks about the Shah as ‘the love of her life’.

With the Shah dead, the family was able to move back to the US. They settled in Williamstown, Massachusetts, largely because Reza – now heir to the Peacock Throne – was at college there. Leila enrolled at a nearby school and kept a low profile. In 1984, the family moved to Greenwich. She spent some time at the United Nations school in New York, graduated from Rye Country Day School in New Jersey and spent a short period at the Ivy league Brown University, Rhode Island, where she studied literature and German and Scandinavian philosophy. She took an anthropology course and read some Islamic theology. She spoke English and French fluently, as well as Farsi, her native language. She also picked up some Italian and some Spanish. According to some, she graduated in 1992, others say she dropped out early. Around this time, she began to complain of severe stomach pains. Her mother said that while at university she ‘began to show signs of fatigue’.

One family friend saw her regularly between 1987 and 1994 when, every summer, the Pahlavis would return to Cairo for a memorial service for the Shah.

Did Leila ever raise her voice, argue, shout like a normal teenager?

‘Never,’ he says. ‘She was always an angel, Very sweet, very sensitive, very dutiful. I never heard her arguing with anyone, least of all her mother. She would always do what she was asked, even if she didn’t want to.’

Everybody says the same thing about Leila Pahlavi. ‘A wonderful girl’, ‘deeply kind’, ‘fantastically sensitive’. No one says a bad thing about her, other than she was a little withdrawn and tended not to talk much.

It is equally difficult to get an idea of her lifestyle. There is no structure to form a narrative around. The princess led a life of discreet, largely uneventful leisure. The ambassadors’ parties, the Swiss skiing trips, the 16th arrondissement dinners, the gallery openings, San Lorenzo’s, nights at Tramps, the friends with Ferraris and surnames that include Von and titles that include Monsieur Le. As a Pahlavi princess, Leila couldn’t have worked even if she had needed or wanted to. Instead, she spent her time helping her brother run the Mihan Foundation, an Iranian cultural organisation dedicated to building links between Iran and the West. She liked poetry and spent entire nights translating the works of the mystic Persian poet Rumi into French or English. She liked to manipulate photographs on her computer, a ‘talent’ she attributed to her sensitivity to natural and aesthetic beauty. She had a few boyfriends, one of whom was serious. The relationship apparently ended badly. She went skiing. She spent a year in California. She had a flat in New York – where she had two pet dogs – and stayed with her mother or grandmother when in Paris. When she was there, she would visit the Persian antiquities gallery in the Louvre.

She gave the occasional interview to gossip magazines which reveal very little other than a sensitive, slightly dreamy, quiet and kind woman who spoke a lot about her mother – ‘I feel so close to her… I have such confidence in her wisdom… I can tell her everything… but I know she is full of anguish for her country, for all Iranians, for us’, and she referred constantly to Iran: ‘There isn’t a day when I don’t think about my homeland, about the snowy summits in the pink of dawn, about our family holidays on the island of Kish in the Persian Gulf, about the rose gardens of Shiraz.’ The Shah, she said, had been a good man who had been lied to by his advisors. He had always had the best interests of his country at heart.

She liked watching football. She ‘liked to meet people from all trades, class and origins’. She smoked Marlboro Lights, though not too heavily, and drank occasionally and in moderation. And was desperately, deeply, unhappy.

Quite when things began to go wrong is hard to say. By her late twenties, she was painfully thin and spoke to friends of a variety of ailments: headaches, muscle and joint aches, chronic fatigue. In pictures from the early part of the decade, there is no apparent wasting. By last year, she is obviously seriously unwell.

Last summer the princess went to Cairo for a ceremony to commemorate the 20th anniversary of her father’s death. She was thin, but complained to an old friend that she was overweight. ‘I’ve put on far too much. I’ve got an extra five kilos on me. I am like a pig now,’ she told him.

A few weeks later, she was in Paris to model for Valentino. While there, she gave an interview to a French magazine. She spoke at length about the Shah’s death, and how it had affected her, but said that she knew she had to ‘get on with her life’.

‘These days, I have to take my life seriously. Many different avenues are open for me, but I struggle to concentrate on one alone. Quite a few people can’t see that, behind my title, I am still a woman who can love. I know that it is not easy to find an ideal companion, especially as I have the example of an exceptional father and two brilliant brothers always before me. I know that I am open, tolerant, warm, but I am strict on certain principles and values, such as keeping your word once you have given it, and punctuality.’

As ‘lonely hearts’ ads go, a full-colour spread in a French glossy isn’t bad.

Soon afterwards, she checked in to The Priory, the rehab clinic favoured by the rich and famous in Roehampton, south London. It was not the first time she’d had such treatment. Stays in luxury detox centres in the US had failed to help her. According to some, she had rejected several clinics as insufficiently luxurious in comparison with those she was used to in the US.

In November, Leila’s grandmother, to whom she had always been very close, died. She was buried in Passy cemetery in Paris. ‘At the funeral, Leila looked worse,’ says the friend. ‘She was talking about her weight again. But she seemed OK. She wasn’t obviously depressed.’

In January, Leila was back in London. ‘She called me on the mobile. She was always very casual. “Hi, it’s Leila”. We went out for dinner at Yass, [the Iranian restaurant in South Kensington],’ he recalls. ‘There were four or five of us. We all ordered kebabs and rice, the classic Iranian dish, but she wanted nothing. She told me she’d eaten a single egg in three days. I told her she was going to kill herself, but she brushed it off. The next night, we went to a friend’s house and she just drank tea and then went on to San Lorenzo’s. She ate nothing there either, but she went off afterwards to a party that a tycoon type was throwing, and told me later she had had a great time.’

In February, Leila went skiing in Switzerland. By March she was back in the UK. She told friends she was planning to sell her apartment in New York and buy somewhere in London, and keep busy by setting up a business dealing and exhibiting Persian art. A friend of her mother’s had offered to fund any venture she wanted to try.

It never happened. She was dead within three months.

Seconal is the commercial name of the barbiturate quinalbarbitone. Its generic name is secobarbital. Its chemical name is sodium 5-allyi-5-(1-methylbutyl) barbiturate. Users sometimes call it Seccy, Seggs or Seggy. It is highly addictive and, because a slight overdose is often fatal, is extremely dangerous.

Like all barbiturates, Seconal is a non-selective central nervous system depressant that is primarily used as a sedative hypnotic. Take one or two and everything – breathing, heartrate, thinking – slows down. In lesser doses it works as an anticonvulsant. In larger doses, it is used for the short-term treatment of insomnia.

Rohypnol, or flunitrazepam, is known as the ‘date-rape drug’. It is a powerful benzodiazepine and thus related to Valium. It is an effective sleeping pill which comes in 1 and 2mg tablets. The maximum therapeutic dose is 2mg. Its effects are fairly long-lasting, but flunitrazepam taken alone is unlikely to produce death, even if an overdose is taken.

Palfium or Dextromoramide is a very potent painkiller related to morphine and thus heroin.

Both Rohypnol and Seconal are physically addictive, and use of all three drugs rapidly builds up tolerance. Which is why when she died Leila was swallowing 40 Seconal and 30 Palfium or Rohypnol a day. The postmortem also revealed traces of cocaine in her system.

It’s difficult to stay when she started taking the pills. Severe stomachaches – like those she complained of after leaving university – are a symptom of barbiturate withdrawal or dependence. The ‘fatigue’ noted by her mother in the early-90s might have been a symptom or a cause of the drug abuse.

We know that Mangad Iqbal, a doctor at London’s exclusive Brompton Medical Centre, told the coroner’s inquest into Leila’s death that the princess had said she had been addicted to Seconal since 1992. That may or may not be the truth, as addicts exaggerate a lot. Dr Iqbal wrote out prescriptions for several hundred pills for the princess over a two-year period.

The first was for 30 Seconal and 90 Rohypnol. It was written for Leila after another of Dr Iqbal’s patients described her to him. A second prescription for 100 Seconal and 180 Rohypnol was written out a few months later – though the doctor had still to meet the princess – at the request of her chemist in Pimlico.

On 10 December last year, after Leila’s trip to Paris and her stay in the Priory, Dr Iqbal actually got to meet his new patient. However, though she came for an appointment she failed to answer questions about her medical history and refused a blood test. ‘She said she had been diagnosed with ME (chronic fatigue syndrome), was in constant pain and unable to sleep,’ the doctor told the court.

Dr Iqbal gave her another prescription for barbiturates and, three weeks later, wrote out one more after a telephone conversation. He then heard nothing from her until the beginning of 2001. On 9 February, around the time that she was having no dinner in Yass and San Lorenzo’s, and again on the 16 May this year, further prescriptions from Dr Iqbal were forthcoming. This time for 100 Seconal, 120 Palfium and 150 Rohypnol tablets.

In addition, according to the police, anyway, the doctor prescribed 120 Palfium and 100 Seconals on each of two further occasions on 10 May and 16 May.

On 6 June, according to Dr Iqbal, the princess had another consultation. While he was out of the room, she had taken prescriptions from his desk. Apparently, the princess said she had ‘lost’ the prescriptions he had given her two weeks earlier.

‘I said I would send her [new]prescription to the chemist directly. I wanted to check her eye, while she was there, and I had to get some equipment from downstairs,’ Dr Iqbal told the court. ‘I told her to sit down and I went downstairs, but when I came back up she had gone. There were prescriptions on the table that she had taken. There were five in total. Two were for her and two were for other people. The two for her were for Seconal and Rohypnol… There were also two more prescriptions for Rohypnol and one for Palfium, which is an opiate.’

He contacted the pharmacy normally used by the princess but did not tell the police.

Leila, one way or another, had got hold of prescriptions for more than 1,000 pills of extremely powerful controlled drugs in slightly more than three months.

The Leonard Hotel is a very exclusive hotel. It is on Seymour Street behind Marble Arch in the heart of the district most favoured by London’s community of Middle Eastern exiles. The streets around are full of shops – Kuwaiti, Lebanese, Iraqi, Iranian – selling hummus, falafel, ful, pulau, chalo kebabs, shwarma kebabs. Old men sit outside cafés sipping coffees, smoking narghiles, as the red buses and black cabs file past.

The hotel itself is a beautiful 18th-century townhouse tastefully converted into suites (starting at £200 and going up to £550) and five private apartments. Leila’s one-bedroom suite, room 15, cost around £1,300 per week.

On 5 June, a day before Dr Iqbal claims his patient stole the prescriptions, Louis Martinez, a room-service waiter at the hotel, had, while delivering a club sandwich, found the princess lying on the floor with a telephone cord wrapped around her neck.

‘It was like she was drunk, as she could not speak properly or get herself up. I could not speak to her. I carried her to bed in the next-door room and asked her what happened. She said “I took so many pills.”‘ There is a sense of inevitability about what happened next.

The princess’s mother had been concerned about her daughter for some time. She rang Hourieh Dallas, an Iranian exile in London and an old friend, and asked her to keep an eye on Leila. ‘[The princess] was a depressed person. She often suffered from mood swings and was concerned about gaining and losing weight,’ Dallas told the court. ‘She was very unhappy.’

On the evening of Sunday 10 June, the Empress rang Dallas again and asked her to go to the hotel and meet a Harley Street consultant psychiatrist, who had been treating the princess for some time. They rang up to the room 15, but there was no answer. When their knocks on the door went unanswered, the hotel manager let them in with a master key. Dallas opened the door and went in first, in case the princess was naked.

‘She was on the bed with the covers pulled over her,’ Dallas said. ‘I thought she was sleeping, but the doctor told me she was dead.’

Her blood contained a quinalbarbitone level of 27.3mg per litre, five times the potentially fatal level.

There were 16 types of medication in the bedroom including 96 Palfium pills, 40 Rohypnol and 103 Seconal. There was also a small amount of cocaine. There was no note, just an undated scrap of paper with some scribbled poetry. In a drawer was the picture of the Shah and her mother and the children all watching television in the palace in Tehran. Leila, the little princess, is sitting on her father’s lap.

There was some talk of suicide. But nothing indicates that Princess Leila Pahlavi deliberately took her own life. She wouldn’t have known she was dying. The sounds of the traffic and the builders’ shouts and the planes flying into Heathrow, and all the grind and rattle of a summer’s afternoon in central London would have dropped slowly away, muffled and thickening as the drugs took hold and her metabolism softly and gently slowed.

And then all would have been quiet. Quieter, even, than the drive along the smooth, tarmac, tree-lined avenues with the branches flickering light and dark across the darkened windows of the big cars, as the little girl looks up through the rear window, through the reflection and the leaves, and sees the white walls and the brown slopes of the Alborz mountains and the blue sky of Tehran in the winter.