Belle & Sebastian, Brixton Academy London

Even a gang of avowed fame-avoiders like Glasgow's Belle & Sebastian have to go out and meet their public occasionally. Tonight's sold-out show, part of a tour to promote nothing in particular – their next album, an expansion of their largely rejected soundtrack for Todd Solondz's film Storytelling, isn't out until June – is part of a global jaunt lasting a few months.

The first "ironically" held-aloft lighter makes an appearance before this famously erratic band have made their first mistake of the night, to more ironic cheers, during the opening "Sleep the Clock Around". It's immediately obvious, too, that any errors won't be obscured by volume. Though Stuart Murdoch's ensemble have never been known for their rowdiness, the sound is ridiculously quiet, despite the audience's frequent entreaties to "Turn it up!". It'd be nice to hear them properly.

The effortless Northern Soul stomp of "Dirty Dream Number Two" rushes by, providing nearly audible evidence that this once much-derided band are capable of switching styles with amazing facility. "I'm Waking Up to Us" is revealed as a close relative of the Justin Hayward-sung "Forever Autumn", from Jeff Wayne's ridiculous 1978 War of the Worlds album; a new song, "Wandering Alone" strongly resembles the Roy Orbison copy that the Mavericks sold millions of a few years ago.

As ever equipped with what looks like the proceeds of a raid on a school music room, including recorders and various percussive implements, the very extended line-up (there's more than a football team up there, almost a rugby XV when the string section joins in) seem close to nailing it. A superb version of "Don't Leave the Light On, Baby" finally admits its debt to Seventies classic soul.

Then, quite absurdly, a steel band appear, to perform a medley including "No Woman, No Cry" while the star turn leave the stage for a half-time talk. It's unlikely that this is Murdoch's idea of adding local colour (though it is possible – they once played the theme from televised snooker when performing at that shrine of the green baize, Preston's Guildhall). But it certainly succeeds in destroying the moment.

The second set never recovers from this blip. "The Boy With the Arab Strap" (aka the theme from Teachers, heh heh) sets the crowd swaying, but the great "Lazy Line Painter Jane" receives a desultory going-over, complete with a contrived, rock-out ending. Not even the knowing, novelty hit "Legal Man" can elevate the mood. A clumsy encore of the Only Ones' "Another Girl, Another Planet" proves only that Belle & Sebastian possess impeccable taste.

Yet their propensity for self-destruction remains intact. They could be one of the all-time greats. They have some wonderful, beautifully structured songs, yet they insist on feeding them through a sound system so feeble they're either insulting their ever loyal supporters, or perhaps showing a parental concern for their hearing. There's nothing esoteric about their appeal, and they're certainly not plodders in the vein of Travis and Stereophonics. They should sell millions, but their own contradictory nature makes that prospect seem more distant than ever.