Finally it's here, The Bodyguard's Story. While the world wept in August 1997, unwilling to believe that something as prosaic as a car accident could have killed Diana, Princess of Wales, the one man who knew the truth was in a coma in a Paris hospital, with only half a face and no tongue.

The extent of Trevor Rees-Jones's injuries was only one of many rumours that would be confounded in the days and months following the crash in the Pont d'Alma tunnel.

Although his face had been completely flattened, the rest of him had emerged remarkably unscathed thanks to the airbag in the front passenger seat. Quite early on he was told by the French surgeon who put him back together that he'd be playing rugby within the year. And he was. Two and a half years on, you have to look really hard to see anything wrong with his face at all.

However, the idea that Rees-Jones holds the key to what happened during the early hours of August 31st, 1997, is less easy to dispel. His memory of the events leading up to the accident is as clear as a bell but he remembers nothing after getting into the car.

We met in a London hotel room. I arrived early and walked in on a family conference. I told his mother how much I admired the way the family had coped - with the press, particularly with the pressure from Mohamed Al Fayed. It was nothing special, she said: "You cope in the only way you can, the only way you know how."

The way Trevor Rees-Jones has coped is to be scrupulously honest, scrupulously fair. The French judge recently turned down his case against the Ritz but he is not going to appeal, nor is he going to sue anyone.

The proceeds from the book will cover the legal fees and should provide a cushion for the future, should anything go wrong. Though he would never have gone into print at all had Mohamed Al Fayed not turned on him. He needed to put his side of the story.

Journalists are surprised by two things when they meet him, he said: "(a) that I'm not breaking down in tears every five minutes and (b) that I'm not up in arms and badmouthing my ex-employer, Mr Fayed, every five seconds."

The first is easy to understand. As Rees-Jones himself admitted, when it comes to emotions, he's "not that kind of bloke". But the second seems unbelievable. Mohammed Al Fayed - (always refereed to as "Mr Fayed") has not only accused his former employee of dereliction of duty, but of being part of a conspiracy - with, among others, Prince Philip - to murder Dodi and Diana.

Trevor Rees-Jones is clearly a man to whom duty and loyalty are paramount. He has no personal feelings about Diana, beyond describing her as "a woman you could take down the pub, and from me, that's a pretty high compliment". He rightly accepts no blame in her death, though he still feels responsible that it happened "on my shift".

He admitted that he had always been aware of Al Fayed's unsavoury nature. "You'd be blind not to know if you read the press," he said. However, he added, in the personal security business you have to be mercenary. "If you just worked for people you liked - I'm not saying I disliked him - but your client list, if you like, would be very, very small."

He returned to work for Al Fayed six months after the accident, resigning only after he had been conned into giving an interview with the Daily Mirror. Asked why he didn't resign earlier he said he saw no reason to. I suspect he stayed as much for the camaraderie of "the lads". Trevor Rees-Jones is a team player.

From the moment we met, the heavily built, but good-looking former paratrooper was distant and wary. Only when his co-bodyguard, Kez Wingfield, joined us did he loosen up. They were together in France, on the boat, in St Tropez and Monte Carlo and at the Ritz. Kez didn't want Trevor to go in the car with them. He had noticed that his friend "wasn't in love" with the driver, Henri Paul, and was stressed out with arguing with Dodi, so he thought it best that he go instead. He still wishes he had.

They were a good team. Everything had gone so well that summer, in spite of Dodi's not telling them any of his plans much of the time, a cardinal sin from the point of view of personal security staff, who need to check everything in advance, from parking to hospitals. "We worked very hard," Rees-Jones said, "and we did a great job considering we were short-staffed on moments. I don't want that tragic end to cloud the whole previous six weeks which I hope everyone enjoyed and, on the security side of things, was a success."

The Bodyguard's Story is less circumspect in its language than Rees-Jones in the flesh. Unlike many such "co-written" stories, it is not written in the first person. A largely practical decision, he said, because he knows little or nothing about many of the events surrounding the accident. The book pulls no punches when it comes to Mohamed Al Fayed - it's no wonder he tried to get it banned. There have been threatening phone calls. Rees-Jones's only concern is his mother.

"I'm big enough and ugly enough to look after myself."

Only little people, he said, resort to phone calls, and they're not worth worrying about.

We may not have the details of those final 10 minutes, but other than that - the romance, the cockups, the horror - it's all there. Far from being a playboy, Dodi turns out to have been rather clueless - the real unanswered question being how he ever managed to land Diana in the first place. Though unwilling to go into details, Rees-Jones said the pair did "click" and "enjoyed talking to each other". Though whether it would have continued beyond the summer, he "wouldn't like to speculate".

You can understand Al Fayed's exasperation. However, the lengths "the Boss" was prepared to use to get Rees-Jones to subscribe to his conspiracy-of-the-century view of what happened make chilling reading.

Apart from the ill-advised interview with the Daily Mirror Rees-Jones never gave Al Fayed what he craved: proof of the alleged engagement, proof of the paparazzi's guilt, let alone confirmation that Diana's last words had embraced his son.

When it became clear that Rees-Jones wouldn't play ball, Al Fayed turned on him. How did it feel to be manipulated, lied to, badmouthed and betrayed by someone to whom you had given complete loyalty?

"I find it disappointing that he has attacked myself and Kez professionally. I don't feel there is any reason to do it," he said.

"My wish for him is that perhaps he can do the same as me - put this into the history file, accept the judge's conclusion, accept the investigation's conclusions. It is a tragedy and it's a terrible thing that's happened, but if I was still thinking that way every day of the week, then I wouldn't have made any step forward at all and I may as well have perished in the car accident."

Doesn't he hate Al Fayed? "No."

Didn't he think that Al Fayed was ultimately responsible for what happened by creating a climate of fear that meant the bodyguards felt unable to check that the change of plan on how to leave the Ritz had his blessing? Kez nods in agreement. Beside him on the sofa Trevor's face remains impassive.

"That's your opinion. My opinion is, I just consider it as an avoidable accident," he said.

"That a mistake was made by Henri Paul to get behind the wheel of the car when he knew that he had been drinking. Not declaring either to us or to Dodi that he wasn't fit to drive. That was the mistake. I have accepted the finding. It was a simple drink-driving accident caused by speed. And that is what it was."

The Bodyguard's Story is published by Little, Brown & Company; price £16.99 sterling