John Martyn, Barbican, London

"Burble burble burble ... President Bush ... flffle mffle wffle ... 'kin 'ell ... urgle wurgle gurgle ... I'm trouble too!. Heheheheheh..." So - impenetrably - began John Martyn's contribution to this year's Don't Look Back festival (at which notable figures give complete performances of the recorded works that, to some degree, made them what they are). He sat centre stage and vast as a range of hills, his one remaining leg cocked on an effects pedal, a Les Paul lashed to his upper slopes like a frozen climber. He then prefaced his performance of his best-loved Solid Air album with 45 minutes of the kind of music that has both perplexed and thrilled his fans ever since that landmark release in 1973. It was both thrilling and perplexing.

Whether or not it was recorded in sympathetic tribute to fellow wayward-traveller Nick Drake, and whether it is indeed the defining classic of Anglo-Scottish folk-jazz-blues it is mooted to be, Solid Air remains one of the most trustworthy records ever made.

So absolute is its reliability, it might have been fashioned by Japanese manufacturers from bonded tungsten and steel, rather than wood, string, air and repeat-echo. Whether you're drunk or sober, in love or out, alone at night or entertaining by day, struggling with tax returns or sitting in a bath, Solid Air never fails to work. Which is a plain way of saying that its emotional, structural and technical integrity is of such a high order that there isn't a context in which it doesn't make life beautiful.

According to its producer John Wood, the singer/guitarist's attitude towards the album has always been ambivalent. Heaven knows why. But we can be pretty sure that it's to do with the record's measured near-perfection. Martyn is profoundly suspicious of perfection, on grounds that it is an illusion brought on by not enough experiential grounding in the travesty that is life.

Still, he got through the album without obvious omission and in completely the wrong order. After all, how disappointing would it have been if he'd done the thing note for note and in the right order? Very, is the answer.

The whole point of Martyn's music is that it is subject at all times to the vicissitudes of the moment. He is a natural improviser. Music seems to flow through him just as time does, reliably but not always appeasingly. After his 45 minutes of cadenced drift in the company of a more sympathetic band than has sometimes been the case since 1980, the acoustic instrument came out ("Ooh, the Bay City Rollers!" he exclaimed at its tartan guitar strap) and he pulsed through "Over the Hill" with bassist Alan Thomson on mandolin. It was extraordinarily lovely. As was his re-pointing of the melody lines of both "Don't Want to Know" and "May You Never". But then he got locked into enjoying the vamped cadences of the latter and couldn't seem to stop. "Gordon Bennett!" Then "the devil's echo machine just died" and he had a wobble, and you began to see how vulnerable this vast, uncomfortable, angry man is. He saved the moment with an aptly still "Go Down Easy", during which Thomson did enough to make sure the recording's original bassist, Danny Thompson, was not missed unbearably. "Solid Air" just flowed.

"I hope you found it as amusing as I did," said Martyn afterwards, unable as ever to enjoy the moment without cauterising it. But he did exit with a smile, perhaps not of triumph or joy, but of some relief. It might have even been pride. He could certainly be proud.