The world knows what happened to the Mercedes and its occupants. In the subsequent collision with a pillar in the tunnel, the car was flattened. Dodi, his driver and Diana, Princess of Wales were killed, Trevor Rees- Jones, her bodyguard, was grievously injured. The world, though, knows nothing of the Uno or its occupant or occupants.
Whoever they were, they wove their car round the wreckage of the Mercedes before heading off into the night.
All that remains as evidence of their presence are a few fragments of tail-light, a scrape of white paint on the Mercedes' right-hand mirror, and witness reports of a small car blocking the path of the onrushing black limousine. One witness account speaks of a "squeal of tyres and then the sound of a minor impact" before the Mercedes smashed into the tunnel's 13th pillar.
Paris police are searching for the Uno, trawling through French car registration records. They know it was made between 1983 and 1989. Perhaps the car's driver was too shell-shocked at the time, and is now too scared, to come forward. Perhaps the driver was over the alcohol limit or was having an affair or was up to no good and was terrified of being caught.
Who knows. What is known is that by driving away, the Uno driver was prepared to commit the serious offence under French law of failing to assist at the scene of an accident. Who is he or she? What sort of person drives around a mangled car with four people inside and does not stop to help or rush to use an emergency phone? Were they alone? How have they managed to keep their secret for the best part of two months? Have their family or friends not noticed what must be the most hunted smashed tail- light and smear of black paint ever?
The Uno is Princess Diana's grassy knoll (the site of Kennedy's alleged second assassin), an aspect of her death that, until the driver is found, cannot be explained. Dallas, of course, was different. From the start, Kennedy was known to have been murdered. He was shot in cold blood. Diana died in the sort of car accident that happens late at night in any city anywhere in the world.
There was no Oswald with his shadowy connections - just Henri Paul, Dodi's driver who liked a drink or two or three, and some chasing paparazzi. And a missing Uno.
WE think Diana was killed through drunken driving. We think Henri Paul was so tanked up when he drove her away from the Ritz that he wanted to show the paparazzi what for, with terrible results. We think. I think. But we do not know. I do not know.
Every newspaper and news organisation, with the exception of the more excitable elements of the Arab media, has decided it was an easily explained crash. Lurid theories about her death abound on the Internet but that is the domain of students in anoraks - desperate, like the fundamentalist Muslims, to pin something on the Satans of the Western security services and their imperialist masters.
Yet people who read serious newspapers and watch serious television programmes still have their doubts. Perhaps in this uncertain world they need to find a perpetrator, they cannot accept that the most popular woman of her time was wiped out with her playboy lover in an ordinary car crash after a night at the Ritz. Their suspicion reflects
another sentiment, that behind much of what happens at the top of our society lies the hand of dark, mysterious forces. We are fed a constant diet of films and novels suggesting that MI5, MI6, theCIA and other sinister groups, are capable of anything. When something happens in real life, we turn to a plot from fiction. Occasionally fiction becomes reality - as in the case of Georgi Markov, the Bulgarian dissident killed by a poison-tipped umbrella, or the kidnapping by Mossad of the Israeli whistleblower, Mordechai Vanunu.
This is what leads many people to suspect something similar occurred with Diana.
It is a recurring topic in bars and at dinner parties, yet barely a word seeps into the press. "She was killed by MI6," said the man across the dinner table." Ho, hum. "No, it was a cabal of right-wingers anxious to wreak revenge on Mohamed Al Fayed for wrecking the lives of Jonathan Aitken and Neil Hamilton, and for bringing down the Tory government," declared the person on my right. Hmm. "I think the security services killed her and I think it deserves looking into," said the man two along. Really? A public relations adviser, an academic, a City banker: three intelligent, sophisticated people united by a nagging doubt.
First, the victim. To get the plot off to a flying start, Diana feared she would be killed. She once, apparently, confided in friends that the security services would dispose of her because she was a "loose cannon".
One episode that conspiracists have seized upon was the way in which Barry Mannakee, her bodyguard, was transferred from her side because he had become "over familiar". Soon after, Mr Mannakee, of whom she was undoubtedly fond, was killed in a motorcycle accident.
Her refusal to be cowed, to go quietly, so the theory goes, had taken her into new avenues, to campaigning for the abolition of landmines to the fury of the United States government and the shadowy arms community. She was becoming ever more closely involved with a Muslim, and not just any old Muslim but the son of the man who was, and is, a thorn in the side of the Conservative-leaning British Establishment. So, she was cut down.
Next, the motive. If the aim was to remove her, to silence her vocal opposition to landmines or to prevent the potential embarrassment of the heir to the throne being embraced by the Fayed family, then the crash was a success.
But only so far. In fact, the landmine campaign acquired an added impetus from her death. And, if there was a fear that she would become a popular alternative to the House of Windsor, why not sit back and let her become entwined with the son of atycoon branded a liar by an official government inquiry? With any luck, her opponents must have hoped, she would have gone and lived abroad with Dodi in California or Paris, and her appeal in this country would have diminished rapidly.
In the end, her death, with its subsequent outpouring of grief, and her veneration as the people's princess, did more harm to the institution of the monarchy than if she had lived. If the intention was to clear the way for the Prince of Wales to marry Camilla Parker Bowles, his long- time lover, that too, appears to have failed, for the time being at least. And if the desire was to seek revenge on Mohamed, why not just eliminate his son?
Finally, the deed. It is here that conspiracists seize on the Uno and other oddities. As a security official in one of the main Paris hotels, Henri Paul would have known intelligence agents. The French secret service relies on people like Mr Paul for tips about who is in town, who they are meeting, what they are doing. As a military reservist he would also have come across intelligence personnel.
Were his contacts with him the night he died? Did they know the route he planned to take? Even if they did, this implies some sort of co-operation between the British and French security services - two organisations that traditionally have been the best of rivals rather than friends.
Besides, in his only known utterance to date, Mr Rees-Jones, the bodyguard, has said it was Dodi's decision to use Mr Paul. According to the bodyguard, Dodi set the agenda, not MI5 or some other hidden assailant, and he set it that night.
Discuss this around the dinner table and back comes the answer: like Mr Paul, the bodyguard will, on his military service, have met security service people. His remark cleverly steered investigators away from Mr Paul.
Suicide by Mr Paul? Unlikely. If so, it was a spur of the moment decision. There is no record of any note, and he was due to meet a woman companion the following day. Being driven around Paris by a drunken depressive on the verge of suicide who only learnt he would be behind the wheel minutes previously does not smack of a carefully planned assassination.
Almost an hour after the accident, Diana was put in an ambulance. A further 43 minutes later, the ambulance reached the hospital 6kms away. During the emergency vehicle's tortuously slow progress, it passed two other hospitals. This inconsistency is easily explained: her blood pressure was low and it is standard for ambulances in such circumstances to go slowly, to avoid braking and sending the patient's blood literally up and down the body. The Pitie Salpetriere hospital was the duty emergency centre that night - and it was better equipped than the others.
Which leaves the Uno. Speculation about the car has heightened with the way in which French police first denied that another car had been involved, then changed their minds. That is the sort of move conspiracists love - a hint of a cover-up, of officialdom reluctantly coming clean. Conveniently, Mr Rees-Jones's memory only extends to the time leading up to the departure from the Ritz. The one man who could talk definitively about this elusive vehicle cannot remember.
If the Uno was part of some plan, how were its movements co-ordinated so that it would be in the right place at the right time? Surely, if it had been hit at all at such terrific speed, it would have suffered much more damage.
Questions, questions. In time - by June next year, they hope - the French police will complete their investigation. Their slowness and refusal to indulge in the British practice of holding detailed press conferences has only added to the dinner-party gossip. When it comes, their verdict, especially if it concludes Diana's death was a tragic coincidence of a drunk driver and a fast car, will not assuage the speculation. "As soon as I heard, I knew they had got her," said a friend last week.
Diana thought "they" would get her too. She once reportedly confided in friends: "One day I'm going to go up in a helicopter and it'll just blow up. MI5 will do away with me." Diana knew, you see. Not even "they" would kill her in public, in a car in a tunnel in the centre of Paris with a witness who lived. Would they?