A year ago, the follically-unrivalled half of the late At The Drive-In, along with a new gang of comrades, took ULU by storm at The Mars Volta's debut London show, providing support to headliners Les Savy Fav. Few who came to sample their new musical wares left disappointed - they were a collective whirling dervish, indescribable and irresistible. As a result, this show at the Mean Fiddler, their first London headline gig, inspired something far beyond mere anticipation.

And perhaps that's the problem. Nothing lives up to the hype; worship someone as a god and their human failings become all too evident, all too soon. The Mean Fiddler is sold out tonight, and the crowd is full of devotees (some carried over from the ATD-I days, others new to the cause) of outspoken, acrobatic vocalist Cedric Bixler and guitarist Omar Rodriguez, whose onstage movement pays crazed homage to James Brown. Let's not fuck about; Cedric and Omar ARE The Mars Volta. Look at the official mailing list flyers, even - do you see the rest of the band on there? It's their baby. Since escaping from the unhappy line-up of heroically cult emo/hardcore pioneers ATD-I, this is what they've been working on - a fusion of jazz, reggae, drum n bass, punk and whatever the fuck else you wanna throw in there - chances are you'll find it somewhere in the hectic, tensile blur of The Mars Volta's music. This, my friends, is what Omar and Cedric really wanted to be doing.

And boy, does it show. Every song is an elongated, no-holds-barred foray in every possible direction, trying out every sonic trick they can think of. At first it's exhilarating. There's none of your pre-assembled four chord bullshit or rigorous adherence to a tune; this is freeform in every conceivable sense. But after a while there's a nagging sense that TMV are oblivious of the crowd before of them. They're not performing, they're jamming, playing with ideas in their own sweet time. This is all good in theory, but in practice, it's clumsy, inconsiderate and, most offensively, it's boring.

Each of these songs on record is an intriguing, complex little gem that reveals yet another glittering facet as it turns and progresses. And The Mars Volta are fully capable of exceeding their recorded energy onstage and being the most impressive live act on the planet, a fury of electric, crackling energy and ecstatic satyric abandon - half of us were there last year, we KNOW this. We fucking saw it. There should be no problems here. But there are. An hour later, they're on their encore, and goddamn - they started this song 20 minutes ago, and it's still going! They just won't get off the fucking stage!

There is one basic problem here. Self indulgence. Guys - we know you're enjoying what you're doing for the first time in years - and we're happy for you. Really we are. We like what you're doing - it's great and adventurous and impressive. The long, winding codas would be fine given some structure, a little direction - but ultimately one gets the feeling that TMV feel they're above structure and direction. There are moments tonight when you desperately want to love them - moments of absolute perfection. But all too often these moments are sullied by pomposity, ego and a complete lack of musical self-discipline. And really, a 20 minute encore song is going too far.

Already, the Mars Volta mean everything to a sizeable congregation of people; but they could mean so much more. Is this something they actually object to? Is this ATD-I mark two - another refusal to be sold to the masses and mean something small to everyone rather than mean everything to someone? Is this awkwardness we're seeing? Or just an absolute inability to impose any restraints on themselves? The Mars Volta have the makings of a truly great band, but is that something they are willing to become?