No one likes to be labelled a conspiracy theorist. The term is generally associated with the sort of people who believe the world is run by aliens disguised as humans, or who think the moon landing was a hoax. But it is very important that we do not allow our desire to avoid pejorative labels blunt our critical faculties. Scepticism can be a healthy instinct.

It is unfortunate that most vocal critics of the standard narrative regarding the death of Diana, Princess of Wales, and Dodi Fayed - which was outlined again by Lord Stevens's report yesterday - have not been impartial or, in some cases, credible. This has added to the impression that anyone who believes there are unanswered questions regarding the deaths is foolish, opportunist or both. But this impression is unfair. Whatever one thinks about the quality of the debate on the circumstances leading up to that car crash in Paris, those continuing to ask awkward questions should not all be dismissed out of hand.

Despite the detailed nature of the 832-page report by the former Metropolitan Police Commissioner, a good deal remains unclear. Lord Stevens admits himself that "there are some matters about which we may never find a definitive answer". And there remains enough doubt for rational people to feel uncomfortable. According to a recent poll, a third of the British public believe what happened to Diana was not an accident. This cannot be written off as a fringe belief.

The question of whether anyone had the motive to murder the couple remains unresolved. The report says there is no reason to believe Diana and Mr Fayed were preparing to marry. Mr Fayed's father maintains that there was. But, in any case, this does not alter the fact that the circumstances of the crash itself remain suspicious. A white Fiat Uno was reported as having collided with Diana's car shortly before the fatal crash. No driver came forward to admit involvement. Nor was the car itself ever located. Yet James Andason, a French photographer who had been following Diana and Mr Fayed all summer, was known to own a white Fiat Uno. It was sold and re-sprayed days after the accident. Three years later Andason committed suicide.

Lord Stevens confirmed yesterday that there was indeed a glancing contact to Mr Fayed's Mercedes from a white Fiat Uno as it entered the underpass. But he said he was satisfied that it did not belong to Mr Andason, whose wife claims he was at home on the night in question. We have to take her word for this because all of the closed circuit television cameras monitoring the underpass - which might have supported this alibi - inexplicably failed to record the incident. Does any of this prove that the crash was not an accident? No. But it casts doubt on the assumption that it certainly was.

Many have dismissed the activities of Mohamed al-Fayed over the past decade as those of a father driven out of his mind by grief for his son, Dodi. Lord Stevens hinted at this again yesterday. No doubt the bereaved father is still grieving. But that does not make him deluded. And we should remember that without his campaigning, this inquiry would probably never have been established. Two eyewitnesses, missed by the original French police investigation, were uncovered by the Stevens inquiry. Thanks to Mr Fayed important evidence has entered the public domain. For his persistence in getting at the truth, Mr Fayed deserves credit, not the derision heaped on him from some quarters.

What took place in the Pont De L'Alma underpass on 31 August 1997, might very well have been a "tragic accident", as Lord Stevens describes it. But we should beware the assumption that all the circumstances of this case have now been fully explained and all the loose ends neatly tied up.