Well , she got the first standing ovation for just turning up. The others followed fairly regularly after that, but it was the opener that was perhaps the most deserved. Hobbling on to the Barbican stage with the aid of two helpers, and cocooned in a mink coat (a nice touch for a Womad event) over a voluminous sparkly gown, Nina Simone looked every bit as big as her legend. Indeed, she looked more than a little like the late actor Divine.

Such was her incapacity (or so it seemed) that it was a miracle she even made it to the piano stool. Once parked there by the minders, it looked for a troublesome moment as if she wasn't quite sure what she was here for. You feared for her health (in truth, you feared for many things), but thanks to the healing power of music - or perhaps it was the healing power of Simone's sly stagecraft? - the star somehow prevailed, and she grew progressively more compos mentis as the performance went on.

By the end, in a triumphant, self-referential version of "My Way", she was looking almost chipper. Her voice, of course, is totally gone, but it was truly shocking that this one-time Juilliard School student and classical soloist had seemingly forgotten how to play the piano. Indeed, by any objective standards, Simone simply can't do it any more. And hey, let's give her another ovation just for that!

But if you'd paid pounds 20 a ticket and sat through three hours of world music before she came on, objective standards would no longer apply. Maybe you'd even hung out in the lobby, taking in the neat Somali tent and the beard- making workshops, or perhaps bought some more hand-made shoes for Christmas, for after all this was Womad. But even after that, could you really say - hand on heart - that Simone was any good? For instance, take the band. In fact please, please take the band, take them anywhere. A quartet of honest but limited musicians led by Simone's regular guitarist Al Shackman, they were - again, by any normal standards - woeful. And let's give them an ovation while we're at it.

At times like these, faced by the lame, the halt, or the irrevocably drug- damaged, the critic is apt to take out his little book of the Spanish poet Federico Garcia Lorca and invoke the holy law of the duende. Simply put, this handy Andalucian principle declares that while some performers may have perfect technique, they can still fail to move you, while others, who may be totally shot to hell, can somehow still make your blood boil with the passion of their presence. But as applied to Simone, even the duende has to throw in the towel and pack it in as a bad job: there just isn't enough left to go on, bar the ovations.

Yet there is still the Simone legend, and the powerful folk memory of a performer who has been one of the greatest singer-musicians of the century, so let's celebrate that. Simone still has a wonderfully warm stage presence, though it is heated somewhat artificially by the megawattage of the inevitable ovations, and the perhaps carefully stage-managed sense that she may be about to peg out at any moment. She even has the grace to treat a drunk in the audience with respect ("What you say, sugar?"), and to dignify his request with the song he was calling for. The material is more than worthy of respect too, although the opening series of "blood-songs" (folk and spiritual standards) was rather underwhelming, given the hysteria of her welcome. She even deigned to sing the latest TV-commercial hit, Gershwin's "I Loves You Porgy", fairly early on, although her voice was far from up to it.

As the performance went on - and it was longer than the brief hour one expected - Simone did, at last, achieve a modicum of her old power. A version of Randy Newman's "Baltimore" was played and sung quite rousingly, and an encore of Brecht and Weill's "The Black Freighter" proved marvellously theatrical. "Mississippi Goddamn", "The Other Woman" (the drunk's request), and even the Beatles "Here Comes the Sun" were beautifully affecting, but the poetry was in the pity as much as anything else. It was the legend we were continually standing up for, rather than the actual performance. But the shards of the past offered more than enough to be going on with. She loved being loved by us, she held out a clenched black-power salute by way of thanks, and she ended by getting us all to sing along to "We Shall Overcome": no mean feat in any circumstances. Whatever the shortcomings, the emotional temperature of the event was close to overwhelming.

More than that, we had, at least, glimpsed the legend. And after all those years of listening to the records, we had finally seen Nina Simone. That alone must be worth getting out of your seat for, even once every 10 minutes. As long as you don't have to do it again.