It's been over a decade since Loose Tubes, the collectively-run British big band, called it a day. The Tubes (who brought the world Django Bates, the Argüelles brothers, Iain Ballamy and many others) played an all-original book and their independence blasted fresh air through the UK scene. That legacy hasn't looked like spawning a natural UK-based heir - until now.

With 32 members, many of them symphony players, Io are even bigger - and that part of Ronnie Scott's that wasn't occupied by the band on Sunday night appeared to be mostly occupied by their nearest and dearest. But this wasn't just a party for a bunch of musicians and their families. Though the orchestra's potential jazz-soloist strengths are muted as yet (Bates, Ballamy, Julian Argüelles and others were developed soloists by the 1984 launch of the Tubes), the rich textures and sharp contrasts in the ensemble writing and playing are already remarkable.

When jazz ensembles use string sections, they're often bolted on, politely adding sonorities to the same riffs. This ensemble plays as a cohesive whole, the strings slamming exuberantly into passages written like brass parts, before unfolding more stately classical figures over which a tenor-sax smokily weaves, or languorous sustained chords behind squawky free-improv flute. The riff-writing for the funky parts and the melodies for their shots at Latin jazz were the most predictable parts of a set bustling with surprises, though a propulsive percussion section lifted everything with a beat in it, predictable or not.

But Io's promise was truly revealed in the more open pieces, which often began impressionistically with quiet flute or clarinet figures, or north African cadences over whispering strings, and through which the band would build layers of intertwined lines, the percussion intensifying behind them. A new generation of classical players whose ears are open to jazz's balance of discipline and individual spontaneity gives the ensemble interplay real vitality. As with George Russell's and the late Gil Evans's bands, the effect is to make even the written parts sound newly conceived, and the unjazzlike harmonies set fascinating challenges to soloists.

The show suffered a half-hour hiatus in the second half when a member of the audience went down with a possible heart attack, but the band finally came back with redoubled force.