Weird and exotic things have a habit of tiptoeing up the M4 from Bristol, and the latest is Ilya. Part band, part high-art concept, their debut album They Died for Beauty proved an instant intoxicant for critics. Ilya's sumptuous track Bellissimo has just been bought up by cosmetics monolith Revlon for a worldwide advertising campaign, and you sense that this is an outfit on a roll.

This show was a somewhat experimental toe in the water before Ilya hits the road in May, but the relaxed atmosphere of a half-empty Ronnie's on a sleepy Sunday night made a perfect fit with their languid beats and daring stylistic inventions. Maybe they set out with the intention of making life tough for critics, because their music comes close to being indescribable. There's something like trip-hop in there somewhere, but there's also Edmundo Ros-style dance music, cool jazz, modern jazz, Hungarian dances and grand opera.

The core of Ilya is guitarist Dan Brown, writer/bassist Nick Pullin, and singer Joanna Swan, with the sound filled out by extra violin, mandolin and trumpet, plus a devious palette of samples. Their "act" is just them in the process of making music, though Swan is one of the more striking vocalists you're likely to see. She stood centre stage in a black chiffon nightgown with red feathery trimmings, hinting at bottomless decadence hidden behind suburban net curtains. It made you wonder what would have happened if celebrity madame Cynthia Payne had become a torch singer.

Her voice has a husky bloom to it that travels comfortably over the cracked rhumba of All for Melody or the Balkan-Hispanic brew of Happy and Weak as if these sorts of hybrids happened all the time. In Quattra Neon, Swan held the microphone further from her mouth to unleash a strident, contralto-ish tone Jessye Norman might have admired. Thanks to her bell-like diction, you can hear all the lyrics too - "Here in paradise, dogs are sleeping on the ice", or "Lavish birds gather, database passengers". It don't mean a thing, but it swings.