Ahead of the 25th anniversary of the death of the Princess of Wales, a former senior British detective has pieced together what he believes happened in the final minutes of Diana’s life.

In a new one-hour documentary, Diana: The Ultimate Truth, former police detective Mark Williams-Thomas forensically examines the wider circumstances surrounding Diana’s tragic death on August 31, 1997, including evidence he says is fresh.

Williams-Thomas, who left the Surrey Police in 2000 after stints in major crime and child abuse to pursue documentary film making, looks at the conspiracy theories, speaks to Diana’s friends and interviews key British and French investigators and forensic experts.

Among them is a witness to Diana’s fatal accident, French photographer Grigori Rassinier, who reveals his experience publicly for the first time. There is also Dr Frederik Malliez, who went to the dying princess’ aid, and police photographer Matt Sprake, who examined the crash scene.

Williams-Thomas travels to Paris and re-enacts Diana’s fatal journey as he tries to determine whether any of the foul play theories stack up.

Speaking to The New Daily from Britain, Williams-Thomas said he wanted to deliver a “proper investigation … it’s about finding the truth”.

“It’s about putting together the events and being able to clearly show what occurred,” he said.

“What we did was piece together the chain of events that led up to Diana’s death, and the immediate aftermath.

“What that resulted in was to get into a position where we could make a determination about how that accident was caused.”

Williams-Thomas said it was “quite clear the accident was caused by the negligence of the driver Henri Paul”, who also died that night, along with Diana’s boyfriend,Dodi Al-Fayed.

“Not only had he consumed alcohol, we can be confident from witnesses that he had been drinking in the bar that night,” he said.

‘She would have survived’

Williams-Thomas said while it was clear that the speed with which the car driven by Paul hit the pillar in Paris’ Pont de l’Alma tunnel was the cause of Diana’s death, he believes she could have survived her injuries.

“A number of factors have come into play in terms of medical treatment, but the reality was had she received appropriate medical treatment from the ambulance and at the hospital, she would’ve survived that night,” he said.

Princess Diana was not wearing a seltbelt at the time of the crash. Photo: Getty

So what happened to Princess Diana?

Williams-Thomas he has investigated some of the world’s most compelling cold cases in his documentary work, including a review into the 2007 disappearance of Madeleine McCann.

He has conducted investigations into disgraced British TV star and sexual abuser Jimmy Savile and Oscar Pistorius’ shooting murder of his girlfriend. He also published Hunting Killers, a book that looks at unsolved missing persons cases and homicides.

While he acknowledges the previous British and French investigations into Diana’s death, and the verdict of a five-and-a-half month coronial inquiry, Williams-Thomas’s latest documentary is an attempt “forensically look at all the evidence and come to it with an analytical and evidentiary conclusion”.

As well as those involved with the Paris car crash and its aftermath, Williams-Thomas speaks to those who were part of Martin Bashir’s scandalous BBC Panorama interview, and examines whether the threat of fake bank statements fuelled Diana’s increasing paranoia.

It was widely reported at the time she genuinely feared she would be killed, either in a plane or car accident.

Diana had her home debugged for listening devices and confided in close friends she felt unsafe.

But it was the month she spent with new lover Al-Fayed in August that was to be her last alive. She spent days on his super yacht in the Mediterranean before the couple travelled to Paris on August 30.

MI5 conspiracy

Williams-Thomas looks at whether chauffeur Henri Paul worked for the British Secret Service, M15, given that he was found with £1000 ($1730) on his body.

Why did he choose an indirect route after leaving The Ritz hotel around midnight with his passengers, rather than the most direct path back to Al-Fayed’s apartment off the Champs-Elysees?

Did the white Fiat Uno that crashed into Paul’s speeding Mercedes-Benz as it entered the tunnel cause the car to veer into the 13th pillar?

Was the British establishment or the royal family involved in Diana’s death?

Was she assassinated, as Al-Fayed’s father, Egyptian-born magnate Mohamed Al-Fayed insists?

Martin Bashir’s 1995 interview with Diana has been condemned. Photo: Getty

“Princess Diana talked about dying in a tragic car crash, so that was already out,” Williams-Thomas said.

“She talked about that not only to friends but to her lawyer [and said] that she was fearful and paranoid. She worried that she’d been listened to, she had her whole house swept for bugs. So there was this real paranoia.

“That paranoia increased when Martin Bashir got his interview with her [and] his lies in order to build on her paranoia to get that interview.

“What we set out to do is to cut through where all those conspiracy theories exist and show the evidence.”

Mohamed Al-Fayed has alleged his son and the princess were essentially murdered by the British government and the royal family. 

Diana (partly visible) and the other occupants as the car left the Ritz. Photo: Getty

“It was looked at very carefully by both the British, the French and the inquest and it was shown very clearly that, while not everything could be explained, there was absolutely no evidence to support the allegations that Mr Fayad was making,” Williams-Thomas said.

The eye witness

Photographer Grigori Rassinier, who has never before spoken publicly about the tragedy, was driving behind Paul’s Mercedes as it headed at speed into the Pont de l’Alma tunnel.

Rassinier tells the documentary he saw white smoke, heard horns and saw “a woman” slouched behind the front seat when he came upon the crash.

“It stays with you,” he tells Williams-Thomas in the documentary.

Inside the tunnel after the crash. Photo: Getty

William and Harry

As his documentary is set to air, and deliver what producers hope will be a definitive conclusion, Williams-Thomas says he hopes Diana’s sons, William and Harry, will watch it.

“Princess Diana was a global icon, and remains a global icon for many reasons but none more so than that she achieved so much in her short life.

“I hope they will see how sensitively we’ve conducted the investigation, how carefully it’s been done … been done in a positive way … brought light into the amazing woman she was.”

Diana: The Ultimate Truth premieres 7.30pm, August 28, on Foxtel and on Fox Docos