"Whatgave impetus to myjourneyswas the need to get under the skin of my subject," announced Michael Asher, biographer of TE Lawrence and the subject of last night's True Stories (C4). First of all though he was going to get under the skin of his Bedouin guides and friends with his insistence - close to religious fervour at times - that every word Lawrence had written in The Seven Pillars of Wisdom was the truth and nothing but. This was, in short, the journey as verifying experiment - an expedition designed to test Lawrence's claims against the terrain itself. Most locals believe that he exaggerated his exploits but Asher, an ex- SAS man and experienced desert traveller himself, was sure that by retracing Lawrence's steps he could confirm the absolute veracity of his hero, that the famed detail of the memoir could be matched, stone for stone and step for step.

To this end he set out first to repeat Lawrence's two-day trek to Al Mudawara, the scene of a famous train derailment described in The Seven Pillars. Travelling with his wife and a Bedouin friend called Sabah, Asher soon found himself falling behind Lawrence's schedule. What's more the topography of the derailment site couldn't be made to match up with Lawrence's description, a source of considerable glee for another Bedouin writer he had persuaded to accompany him, in the hope of proving his case. He couldn't - looking in vain for the two-arched culvert Lawrence had described and estimating distances with a flexibility which said more for his loyalty than the accuracy of his eye (he needed to find something 200 metres from a ridge to match the original description - his first site turned out to be 550 metres away, despite his conviction that he was in the right place). "Why don't you face the facts?" said the exasperated writer. "You see he is a human being. He remembers and he forgets." After he had swayed off on his camel (the voice-over described him as "gloating", which seemed a trifle parti pris) Asher promptly discovered some rubble which he took as proof that he had been right all along. "Nobody could deny that this is the remains of a bridge," he said jubilantly, but the camera kept its distance from the evidence just in case you were inclined to disagree.

The real clincher came when they tried to repeat Lawrence's famous journey from Aqaba to Suez, a trip which he claimed to have completed in 49 hours without sleep. It soon became clear that it would take the team almost twice as long to achieve it and only then if they managed to stay awake. Michael's defensive lines crumbled. What you were looking at here was more than a piece of scholarly exactitude (though it was that too) - it was also a battle between what a man wants to be and what he knows he is. For Asher to believe Lawrence's account, he would have to doubt himself and all his accumulated experience. Was he really half the man his hero was? He might have settled for a larger fraction but not quite that. Back in Britain he returned to Lawrence's pocket diaries and discovered that they didn't match the literary version - it had taken him three days to reach Suez and he had slept twice on the way. "Bloody liar," yelped Asher, when he first encountered this incontrovertible evidence that his hero had a talent for embroidery.

All this talk about the veracity of expedition memoirs couldn't help but cast a light over the documentary itself - such films being prone to the concealment of the caravan of air-conditioned vehicles which are generally required to keep a camera crew happy. Had these travellers really ventured into the empty spaces of the Sinai without a single GPS navigation device in case of trouble? And if not, then how seriously did one have to take the sequence in which they worried about getting lost? We may not have really big and splendid myths these days, such as the myth of romantic charisma that Lawrence wrapped around himself like a burnous. But we do have smaller ones - and directors work hard to preserve them.