So it came as a considerable surprise when, in a television interview last week, he hinted strongly that it was reaching a sensational conclusion.
He said the case was "far more complex" than originally had been thought and that Mohamed al-Fayed had been "right" to raise the issues he had in relation to the deaths of Diana and his son Dodi.
Until now it seemed almost certain that his inquiry would largely agree with the French version of their fatal crash in a dreary Paris underpass in the early minutes of 31 August 1997.
The official account concludes that the driver, Henri Paul, the head of security at the Fayed-owned Ritz, had been drunk and on anti-depressants at the time, and that the car, a Mercedes, was likely to be the only vehicle involved. This has never been accepted by either the families of Dodi or Paul - or, indeed, the band of conspiracy theorists Diana's death has attracted.
Now it would seem the Harrods boss, Mr Fayed, who has spent many millions of pounds funding his own investigations, stands on threshold of a vindication of sorts.
The Independent on Sunday has established that Lord Stevens' remarks were a deliberate attempt to prepare public opinion for some shocking conclusions. "People are going to be very surprised about what we have to say," said one senior officer closely involved with the inquiry. But what, among the blizzard of outlandish speculation, could Lord Stevens have found that has led him to upset the establishment version?
Four issues remain unresolved: the reason for the high levels of carbon monoxide inPaul's blood; whether he was drunk; what happened to a white Fiat Uno seen speeding from the scene, and was Diana pregnant.
The last mystery has been reinvestigated by Lord Stevens. It was reported the former Met Commissioner has talked to some of Dodi's friends, who say he called them hinting the couple had good news shortly before the crash.
But the IoS has spoken to Robert Thompson, the manager of Hammersmith and Fulham mortuary, where her body was taken. He assisted in the examination of Diana's remains and has been reinterviewed twice by detectives. He said: "There have been plenty of theories. But there was no indication of any pregnancy... This is based on what was said by the pathologist at the post-mortem. He made a comment to the effect that he did not think she was pregnant and wrote this in his report."
But, as the IoS reported two years ago, there was a degree of cover-up in the days after her death. A senior police source in France told the newspaper that Diana was pregnant. According to the source, evidence to this effect is to be found among the 60,000 pages of evidence from the initial French investigation.
Suggestions that Diana was pregnant at the time of her death have since been rejected by close friends and by the doctor who performed an autopsy when her body was returned to Britain. Yet the former police officer stands by his statement on Diana's pregnancy this weekend.
Another medical anomaly persists - the question of Henri Paul's blood samples, which show varying degrees of alcohol and high levels of carbon monoxide. David Cohen, the author of Diana, Death of a Goddess, which advances the theory that Paul was drugged as part of an assassination plot, asks why the authorities have failed to find evidence of the security chief's drinking prior to the crash.
It remains puzzling why Trevor Rees-Jones, her bodyguard and the sole survivor of the crash, would have let Paul drive if there had been any evidence of his drinking the amount the samples suggest. And the French authorities' refusal to provide DNA testing of the samples has encouraged speculation that there may have been a mix-up, either accidental or deliberate. It was reported last week that Lord Stevens is investigating claims that two MI6 officers visited the morgue in which Paul's remains were held on the night of the crash.
Mr Cohen also believes Lord Stevens has been following up his information that the mystery white Fiat was driven by a paparazzo, James Andanson, who later died in suspicious circumstances. A French fireman, Christophe Pelat, who attended the burning wreck of the car in which Andanson is said to have set himself alight, says he appeared to have a bullet hole in his skull. Mr Pelat, contacted this weekend, declined to comment on whether he had been interviewed by Lord Stevens's detectives.
Mr Fayed's spokesman refused to comment, saying it was "not appropriate" while the process was ongoing. But another source close to the Harrods boss said: "He feels vindicated that Lord Stevens made those comments. Certain sections of the media have consistently tried to undermine what he's said."
Additional reporting by Katherine Haywood
* Was Henri Paul drunk? Blood testssuggest so but also showed very high levels of carbon monoxide leading to speculation of a 'mix-up'
* What about the white Fiat? Investigators hired by the Fayeds say it belonged to a paparazzo, Andanson, who was found dead in a burnt-out car in rural France
* Was Diana pregnant? Some French medics say tests showed she was; post-mortems in the UK found no evidence and friends say it was 'biologically impossible'
* Lord Stevens. Asked to investigate after repeated bungling by the French undermined confidence. His team amassed 1,500 statements worldwide
* Michael Burgess. The Royal Coroner will receive the report in the next few months but an inquest is not expected until next year
* Mohamed al-Fayed. Dodi's father has spent millions in the belief the couple were murdered - because Diana was pregnant by a MuslimReuse content