Just when you might think everything that could be said about the late Princess Diana has been said, something new will emerge to debunk that theory.

Apart from the Daily Express, which is in a category of its own where the former Princess of Wales is concerned, the 20th anniversary of her death, next week on August 31, has already produced seemingly endless articles and programmes in Britain and abroad.

Princes William and Harry made a tribute TV programme about her; another programme caused a row because it referred to her intimate relationship with Prince Charles; OK and Hello magazines are rumoured to be in an anniversary froth; Diana’s dresses have gone on display; and only the other week there was a peculiar programme about whether or not Diana got on with her stepmother, Raine Spencer.

But now the Haifa-born award-winning documentary film-maker David Cohen has come up with a Diana treatment almost guaranteed to be unique.

In Diana, 20 Years On, an updated version of a film he made about her 15 years ago, Cohen revisits the work he did and has produced a story reminiscent of Dan Brown’s novels. 

Conspiracy theories abound, but Cohen, an experienced journalist who has written and updated an accompanying book, Diana, Death of A Goddess (Arrow) which was a best-seller in 2005, will say only that he believes that those he spoke to were convinced that there was a plot to murder the princess.

Talking me through the story on a sultry summer morning in north-west London, Cohen acknowledges that much of what he is about to describe does sound far-fetched — to say the least.

For context, he says, we have to know about a previous film he made about the entirely mad-sounding Order of the Solar Temple. It was a cult for extremely rich people and was virtually ended in the mid-90s when large numbers of Solar Templars died in what may have been mass suicide.

The film was screened on Channel 4 and two days after transmission, Cohen was contacted by a man calling himself Guy. 

“He said, ‘You got it broadly right. You made less mistakes than other journalists. Meet me in Paris tomorrow night outside [the nightclub] Maxim’s. It will be worth your while’.”

A somewhat dubious Cohen duly went to Paris where Guy, saying he had been the chauffeur to the founder of the cult, gave him information purporting to show that the late Princess Grace of Monaco, the former Grace Kelly, had been a member of the Solar Temple. 

Not only that, Guy said, Princess Grace, who died in a car accident in 1982, had been bumped off, because she was trying to leave the cult when the hugely expensive subscriptions threatened to rise.

Using what Guy told him about Grace Kelly’s death, Cohen made another film for Channel 4, which asked more questions than it had answers. 

Guy had told him that Grace Kelly’s car, in which she was travelling when she died, had been “interfered with” — specifically, the brakes — and it emerged that the car which an engineer was sent to inspect after her death was not the car in which she had died.

Cohen and Guy kept in touch after the Kelly film. “Three weeks after Diana’s death, Guy rang me and insisted I come to Geneva to see him.” And in Geneva, Guy related an extraordinary tale. He told Cohen that he knew an “assassin” through his association with the Solar Temple. The assassin asked Guy to launder £500,000 for him, money which he said was half a fee for an “accident”, which he was supposed to arrange. But things had gone wrong, said Guy: the “accident” had taken place as requested by the assassin’s client but when he had gone back to claim the second half of his fee — another £500,000 — the client had disappeared. And the assassin, somewhat peeved at not getting the rest of his money, told Guy: “I was paid the money to arrange the assassination of Diana.”

Did Cohen believe this story? “I was convinced that Guy believed the story he had been told.” Cohen spoke once on the phone to the alleged assassin, but acknowledges that it could have been anyone on the other end of the line. Nevertheless, it was the case that Guy had received half a million pounds to launder, with the expectation of getting a second instalment, and both men told Cohen that Diana’s death had to look like an accident. Neither of them ever knew the identity of the client who had hired the assassin. Another Cohen film was in the works.

The inquest into Diana’s death did not take place until nearly 10 years later but the overall conclusion blamed Henri Paul, the driver of the Mercedes in which she and her lover, Dodi al-Fayed, died. Dodi, of course, was the son of the then Harrods owner and controversial businessman, Mohammed al-Fayed, who also owned the Paris Ritz.
Diana’s bodyguard, Trevor Rees-Jones, was the only survivor of the crash in the Pont de L’Alma tunnel in Paris. Henri Paul, who also died, was said to have been heavily drinking and way over the legal alcohol limit; but Cohen’s film makes the point that Rees-Jones insisted that he would never have permitted Henri Paul to drive if he had believed him to be that drunk.

Diana herself predicted that there was a conspiracy to get rid of her, a fear she confided to the late Lord Mishcon, her lawyer. She had written to her butler, Paul Burrell:

“This particular phase in my life is the most dangerous. X is planning an ‘accident’ in my car, brake failure and serious head injury”. 

Lord Mishcon was appalled by this claim and went to speak to Commander Patrick Jephson, Diana’s private secretary. To Lord Mishcon’s amazement, Commander Jephson gave her fears “credibility”, though it has never been substantiated as to who “X” was.

It may well be that a Dan Brown-esque conspiracy theory led to the making of the film. Initially, Cohen meticulously makes the case against blaming Henri Paul, piling up fact upon fact until the viewer is left questioning, what really did happen?

We see, for example, footage of a supposedly drunk Henri Paul, who was deputy head of security at the Paris Ritz, where Diana and Dodi were staying before the fatal crash. Paul squats down to re-tie a shoelace. He remains, perfectly balanced, for nearly a minute, not swaying or losing his balance as might someone who had drunk too much.

Cohen was contacted after the screening of the first film by the leading New Zealand toxicologist and forensic scientist Dr Jim Sprott. 
Sprott, who died in 2014, told Cohen that the results of Henri Paul’s post-mortem made no sense. He told both Cohen and the coroner that the samples from Paul’s body did not show that he was drunk. He claimed the coroner never replied.

But the sensational core of the film is a claim by an anonymous witness to how the crash happened. This person suggested that Henri Paul was just out of a relationship and that the aggrieved girlfriend had agreed to plant a phial of nerve gas on him, as part of the conspiracy.

The nerve gas could be detonated remotely and, claims the film, this was done as the Mercedes, driven by Henri Paul, entered the Alma Tunnel. This is what caused Paul to lose control of the car and smash into the 13th pillar of the tunnel. 

So, not a hectic car chase by paparazzi, and not a drunken chauffeur. But a vengeful lover and an as-yet unidentified powerful person or agency, who had at least half a million pounds to spare and possibly another half-million on top of that, all to ensure that the one-time loose cannon of the royal family was silenced for good.

In the weeks leading up to the 20th anniversary of Diana’s death, the tabloids have been out-shrieking each other as they attempt to come up with the definitive answer as to what really happened in Paris on August 31, 1997. 

Just last week, a poignant interview with Henri Paul’s father, Jean, appeared to substantiate the wilder claims in Cohen’s film. Jean Paul told the Daily Mail: “Diana was killed and my son was killed. I believe they were both murdered. My son was simply collateral damage of a plot to kill Diana and they killed him as well. I am 100 per cent sure Henri was not involved in this plot. He was too honourable and too honest.”

For Cohen, a Bafta-nominee for a film he made about the Soham murders, Diana’s death remains one of the great mysteries of the world. He says he doesn’t know the answer for sure. If anyone does, they’re not speaking.