SAY secret to most journalists and they begin to slaver. This Pavlovian response is generally healthy because it encourages an eager desire to unearth information that people in power would prefer to obscure. But it is also a desire that can distort judgment.

The longer-toothed breed of reporter (an increasingly endangered species these days) has learned from dire experience of wearily long wasted hours that the dishevelled man in the pub with the nudge and the wink and the empty glass rarely has a secret to sell, only the need for a drink and a feeling of some importance to help pass another solitary afternoon. The woman at the front counter who imperiously insists on speaking to the editor but will make do with the rapt attentions of the shiny-faced new recruit to the newsroom for an hour or six, rarely offers the stuff of front-page exposes. But digging, burrowing away, even when the hunt proves ultimately fruitless, offer the learning tools of the trade.

Time to think and unravel the strands, to consider the facts - if they exist - are concomitants.

Reason and experience teach that real conspiracies are rare. To the bitter man in the pub, the sad woman at the front counter, though, they may feel real. For conspiracies can be comforting. Staring a cold hard truth in the face may be too sore. He was arrested, his wife left him, his dog ran away, not because a bunch of masonic colleagues were jealous of his ability but because he was a drunk with all the charisma of a skunk. She was hospitalised not because of a conspiracy by her family doctor and her niece to steal her fur coats but because she was found wandering naked round Woolworths last Saturday. She cannot believe that, of course, because reality has leaked away through the holes in her brain.

Conspiracies then help those who cannot or will not face up to their own reality. They prefer to dawdle down blind alleys, skite round corners and into cul-de-sacs rather than take the direct route to the truth. So when Mohammed Fayed insists his son Dodi and Diana, Princess of Wales, were assassinated by sinister figures who disapproved of their relationship, we might understand his paranoia. This is, after all, a man sneered at by the establishment to which he aspired, desperately. No matter how hard he tried, he failed to grasp the essential truth, that he might be tolerated, his money lavishly enjoyed, his hospitality accepted with insouciance. But he would never be acceptable: not even as the shopkeeper of Harrods. He was never going to be British, no matter how he coveted nationality or was willing to pay for it.

Last night's documentary, then, allowed Mohammed Fayed to indulge his theory that Dodi and Diana were dispatched by shadowy agents of the establishment just as his son - if not he - were about to breach it from the rear.

Not a single fact emerged to differentiate this conspiracy from any of the others woven by every crank under Christendom. The trouble with conspiracy theories is that you can never totally disprove them. Because they are based not on facts but on faction - strands of truth woven into dense fantasy - the more you attempt to separate the constituent parts, the more tangled and fabulous the tale becomes.

Just as the pub drunk, the mad old lady, refuse to face up to their reality, so Mohammed Fayed escapes examination of his own part in the tragedy by weaving a theory that might suit him like a slack tuxedo but does not fit a fact. It saves him having to ask himself why Diana's safety was entrusted to a drunk chauffeur playing cat and mouse with madmen on motorbikes. He won't face up to that or admit that every ''fact'' he has produced to ''establish'' the truth has been proven beyond doubt to be bunkum or mendacious or both.

But then, perhaps we should understand that this is a man, an extremely rich, very powerful man, still hurting and still grieving for his son. What is harder to understand is by what failure of judgment senior television executives allowed the programme-makers to peddle such incredible and unsubstantiated guff on prime-time television.

The film was to be followed by an hour of chat - to use the word debate is to dignify 60 minutes of delusion and gossip - which according to the programme-makers was intended ''to dispel a collective spasm of coyness by the British media''. What coyness? Unless there is at least a scintilla of evidence to prove that the events at the Pont D'Alma was anything more than an accident there is no reason to run with the programme. The most callow reporter would be gently taken aside and told to forget it. The programme-makers should have been laughed out of the newsroom.

It's a pity, really, that the former MI5 officer, David Shayler, was excluded from the discussion. He had been due to take part in the studio discussion ''speaking from abroad'' to outline the role of the security services in protecting dignitaries. But the Attorney-General reminded broadcasters that an injunction against Shayler prevents him disclosing any information gained for the security services. Shame.

Not that I believe he could have shed any light on the murk that has now been thrown at the accident. Not at all. But his appearance might have confirmed that security service agents are the biggest fantasists of all.

Shayler was one of those, or so he says, who bugged and burgled, inspired by suspicions closer to paranoid fantasy than to fact. He was. Until he sold his story to a Sunday newspaper and vamoosed. It seems MI5 was so bankrupt of talent that it snapped up Shayler after the Sunday Times rejected him as a trainee journalist. Hardly surprising. He probably came back from lunch with a conspiracy story a week. Obviously better qualified then for a job with MI5.