Amy Ray and Emily Saliers can't quite remember the last time they played London. It may have been six years, or seven. Whatever, they tell the packed crowed, it's been too long. Last time out they were the world's most popular female vocal duo; today that position seems even more unassailable. They have their market - good-natured, eco-friendly, Sapphic protest - as sewn up as Limp Bizkit have theirs and the evening's defining moment comes not when the crowd takes over the infectious Closer to Fine, but when former Haircut 100 drummer Blair Cunningham ambles on stage to shake his maracas to Power of Two. "He's really good," chuckles Saliers, "for a guy." The overwhelmingly female audience collapses as if this is the funniest thing it has ever heard and the circle of affirmation is complete.

The duo's vocals are superlative, whether harmonising like the Everly Brothers on You Got to Show or spinning off each other at the weirdest of junctures, as when Ray takes the lead on Shame on You with Saliers providing the magical counterpoint. Together since 1987, they now sing with as much instinct as polish. They fit together perfectly: Saliers's plaintive just-fallen-in-love balm (heartbreaking on Kid Fears) leavens Ray's gruffer tones.

But the stand'n'strum format ensures there is little movement on stage, the lighting is dreadful and - Ray's ill-advised leather trousers aside - the duo and their two British backing women (the roadies are female too) make no concession to stage clothing. Worse, they seem chary of playing new songs, taking great pains to point out that an older one will follow.

This is baffling, for Our Deliverance and Moment of Forgiveness are especially lovely. In turn, this leads to a reluctance to run through the standards one more time: "We'll do Galileo later," snaps a far-from-pleased Saliers. They do, but perfunctorily. In fact, they rather lose the plot towards the end. They exchange songs of luminous beauty for a lengthy, lazy hoedown that one "y'all" after another cannot render authentic. Once a most thoughtful duo, they have become mentally flabby.

· At Glasgow Fruitmarket (0141-287 5511) tonight.