He said: "We're using the latest technology that the French didn't have obviously seven or eight years ago. Even they're impressed by what we're doing.We can use reality television in terms of what the car did, how it performed on that day -- the forensic side of this has advanced massively.'
Video reconstruction will be part of a preliminary report presented to the royal coroner, Michael Burgess, in August. It has yet to be decided whether the film will ever be presented to the public, disclosed the Daily Express, which has tirelessly campaigned for the truth to be told about Princess Diana's shocking death.
Stevens told the London-based national newspaper that it was "quite extraordinary how things are advancing".
"We've got new witnesses and we're going through new techniques and the rest of it and we're just absolutely determined to make sure that this is a thorough job and we're not going to be hurried into it," he said.
Stevens also revealed for the first time how British police, aided by expert forensic scientists, have taken apart the ill-fated Mercedes S280 in which Princess Diana and her boyfriend Dodi Al Fayed had been travelling "bolt by bolt," with every part tested, before being reconstructed again.
French authorities turned the car over to the British several months ago after completing their own probe.
The Daily Express and other newspaper quoted Stevens this week as saying that fresh witnesses and new evidence had been found during his investigation.
Stevens was reported in The Guardian on Wednesday as saying that he met Mohammed Al Fayed, the billionaire Harrod's owner whose son, Dodi, also died in the crash, every two months.
Speculation continues that the Princess of Wales' death was not the result of a straightforward car accident. Some commentators have strongly suggested that the British Royal family wanted Princess Diana out of the way because they refused to accept her marrying a Muslim who would have become stepfather to the eventual heir to the throne. Dodi's father, Mohammed Al Fayed, has gone on record to accuse the Queen's husband Prince Phillip of having had a direct role in the murky affair.
The Stevens investigation was ordered in 2004 by the royal coroner amid an avalanche of conspiracy theories. But Stevens -- who is promoting his autobiography -- assured the audience at The Guardian-sponsored literary festival where he made the comments that 'each and every conspiracy theory' is being investigated.
"We will go where the evidence takes us and I intend to finish the job. It is a complex business and it is taking time," he added.
Diana married Prince Charles, the heir to the throne, in 1981 but the pair separated 11 years later. She was killed with Dodi Al Fayed and their chauffeur Henri Paul on August 31, 1997.
The French probe blamed Paul for losing control of the car because he was high on drink and prescription drugs and driving too fast. But Mohammed Al Fayed has claimed that the British secret service were involved in dirty tricks and switched blood samples so that those examined by doctors did not come from Paul, who worked for him.
* See also 'Public Think Princess Diana Was Murdered' (The-Latest, General News section)