Portico Quartet, Rough Trade East, London

Get yourself down to the National Theatre without delay. There's a good chance that you might catch Portico Quartet busking outside it. To see this genre-defying south London four-piece is to witness four young, very talented instrumentalists crafting sounds like you've never heard. Besides, how many Mercury Prize nominees do you know who still work the streets?

An intimate crowd assembled for another free gig in Rough Trade East, the now award-winning Brick Lane record store. In the heart of London's über-cool east, 50 seated Shoreditch fashionistas, plus a gabble of standing aficionados and casual shoppers, were treated to 45 minutes of songs from Portico Quartet's critically acclaimed album Knee-Deep in the North Sea – with three new tracks thrown in, too.

For those who've never heard Portico Quartet, the band's sound revolves around the hang, like a steel drum only with more of a dull oomph to it. Having picked it up at Womad, Portico experimented – playing with hands and mallets – and eventually made the sound their signature.

The hang is used by front man Nick Mulvey for different effects: to lilt the beautiful melodies of the album's title track; to whip up the mesmeric drones of "Steps in the Wrong Direction"; and to thrash out catchy, infectious hooks as on the accomplished "Cittagazze", where drummer Duncan Bellamy joins in, also playing on a hang, and double bassist Milo Fitzpatrick pats out a rhythm on the body of his upright.

All of which is to say nothing of the majestic saxophone played by Jack Wyllie. For listeners attracted by Portico's minimalism, its gentle chord progressions and its African rhythms, the sax will be the most difficult presence to get used to. That's jazz, they say. But, aside from some astounding, attention-grabbing solos, it's easy to forget that Wyllie is there at all, busting a gut on soprano as the sound is used to such good effect it simply adds another layer to this musically complex whole.

Each performer has the freedom of spirit that his jazz roots demand; yet the sound they produce has a strange, comforting simplicity to it. And if tonight's selection is anything to go by, the band's new material is even better than Knee-Deep. Gone is some of the whimsy that crept on to that album, to be replaced by pared-down hypnotism. Much more attention seems to have been paid to the art of building and releasing tension.

Yes, there were times when the group appeared to lose its way. Elements of timing and tuning were botched. But they always came back together. They always returned to that grass-roots, organic sound that has taken them from the South Bank, via intimate venues like this, to – who knows: Mercury Music triumph?