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And the treats tonight don't just come from the headliners. Fellow Cardiff four (or five) piece, , are on stage when I walk into the venue.

Only a damn fool would forget how many members are in a band while he's watching them, but Islet are so liquid, in every aspect of their sound and being, that counting members while they swap instruments and leap around the stage, exuding joy at the incredible sound they're creating, is rather missing the point.

I see a lot of bands. I have no problem with tradition. I like it when a band plugs in and plays its guitars, its bass, its drums and its vocals in a stirring, melodic and predictable fashion. It's the little conservative in me. That's with a small 'c'. But to see a band shrag whole new structures, envelopes and possibilities out of those self-same instruments, in the fashion of human ley-lines channeling some primal, uncageable force, is like having new dimensions revealed to you.

In all honesty, the last time a new dimension was revealed to me I was rather glad when it was all over. But in this case the ritualistic, mantra-inducing rhythms, the tiny and perfectly-placed splashes of vocal, guitar and tambourine, and the intelligent, restrained drama of it all rather blew a new and welcome hole in my sense of possibility.

Islet come from a time well before rock and roll and their vision stretches way beyond now. There are comparisons to be drawn: Liquid Liquid, Can, Fela Kuti, even. But their sound and approach is fresh as the Big Bang. I was utterly intrigued.

For the headliners, this is the first date on a long tour to support their fine new album Romance Is Boring. The songs - as you might expect - are like little dramatic wrestling bouts of sound between almost-hidden riffs, playground spangles of glockenspiel, delightful appearances by flute, violin and keyboard, and Gareth's scattergun lyrics.

For certain Los Campesinos! cram more words, more musical ideas, into every moment of every song than their peers manage across whole albums. It can be confusing, but it's also dazzling. There is no danger of boredom or stupor, here. None at all. These are songs for our times, for our over-stimulated mindsets, maybe.

Given the high percentage of songs from the new album, some early moments in the set sound a little shaky. But where there is rust, it's charming. These songs are so new the band needs to ease the stiffness out of them. No matter how many times you rehearse a song in an industrial unit until the hair metal band knock on the door to hint that your time is up, the song behaves differently in front of an audience.

However There Are Listed Buildings sounds like it's always been part of their vocabulary. It's like watching a child get on a bike without stabilisers for the first time. There are initial wobbles, but they soon gain confidence and assuredness. Half way through the set, they're speeding along none-handed, filling us with joy.

At their heart, they're a band with more personalities and personality than they're given credit for. The fault line at the heart of the band from which all this fascinating music quakes is between the fierce, proto hardcore riffs that rip from Tom's guitar, and the sweet melodies they're able to drape around them.

I love the awkwardness at the heart of these songs. They don't kow tow to a dancefloor, purloining breakbeats or modernistic rhythmic tics so that they can convert those seeking a mantra to lose themselves in. Los Campesinos aren't a journey along Kraftwerk's Autobahn where the brain is hypnotised by the regularity of sound and motion; they're all Alpine hairpin bends, fatal drops on one side, head smashing boulders tumbling down on the other. You're shaken awake, and alive, at every handbrake turn.

It's exhausting, but thrillingly so.

I imagine they're terrified of boredom. It might make for a hyperactive experience and a wall of complex, unfathomable sounds, at times, but I'd rather that than a paucity of ideas disguised as insouciance.

Dilettantes trumpeting the Emperor's New Beats are going to hate Los Campesinos!, and I love them all the more for that.

Our focal point (when I'm not - and I'm just being honest, here - being bewitched by Ellen hidden at the back) is Gareth. Tom is mostly responsible for the shapes of the songs that confuse and bewonder us; Gareth emphasises their humanity. And he does that with a refreshingly honest self-deprecation. But there is also fury and frustration, here; eloquence and verbosity; a thousand and one compelling contradictions.

The amount of people stood around me who are singing along is testament to the fact that there are a significant number of people in whom Gareth's words resonate. I am one of them, too. But as someone who was having kittens trying to remember the names of the seven band members in advance of the interview, I think I'll stick with Louie Louie for the time being.

You, Me, Dancing has been their most successful and recognisible song, so far. Despite Gareth's claim that the way they played it was akin to forgetting that your mum is called 'mum', it sounded bloody great to me. And it didn't stand alone.

This was a very fine gig. By the middle of this tour, they'll be pulling wheelies.