Portico Quartet, Ronnie Scott's, London

"Has anybody here seen us busking?" Not a question you expect headliners to ask at this mecca of UK jazz, the preferred venue for US greats to hold court with their residencies.

These four south Londoners have taken over, token representatives of the genre on the Mercury Prize shortlist, with the winner announced today. Not that many aficionados are happy with such a description, for Portico Quartet mix world-music rhythms, Steve Reich minimalism and the sonically open minds of jazz's new generation to create the cosmopolitan, airy feel of their debut album, Knee-deep in the North Sea.

With their talents set to be displayed at this week's ceremony, they have already discovered this century's first original instrument. A dimpled wok with its lid on, the Swiss-devised hang is a nuanced take on the steel pan. Yet while the affable Nick Mulvey sits stage front, his three flying saucers rarely dominate the sound.

Rather than being a collection of soloists – typical for the improvisation game – Portico live as well as play together, making them more like an indie group. They follow melodic lines that build in intensity before exploring variations, usually led by the mellifluous tone of their soprano saxophonist, Jack Wyllie. Milo Fitzpatrick's funky double bass leads "Dawn Patrol", supported by liquid steel-drum tones, while "News from Verona" features a skittering rhythm augmented by ringing gamelan notes from the hanghang (plural).

When this instrument takes a melodic role, as on "Zavodovski Island", the drums provide a more terse, pulsing pattern in this urgent spy theme. "Cittagazza", meanwhile, suggests sped-up film in a documentary on urban life.

Discovered on the streets, the four-piece are eager to entertain, so regularly miss the opportunity to fully express themselves. Moments where the group let rip remain politely truncated, while too many numbers peter out to an unsatisfying close. Apart from the woozy morning-after feel of the album's title track, there is little emotional variety.

They closely mirror the Penguin Café Orchestra, but without their focused eccentricity. "Prickly Pear" is more of a fruit salad, while "Monsoon: Top to Bottom" ought to be renamed light summer shower. There is much more to come from this young outfit, but for now the prize should remain just out of reach.