The winner of the Nationwide Mercury Prize 2008 is revealed on Tuesday. The contest’s worthy efforts to find the year’s best British record, regardless of genre, industry hype or Simon Cowell, has thrown up some intriguing chalk v cheese face-offs. Eight years ago Nicholas Maw’s Violin Concerto slugged it out with Death in Vegas and Coldplay; in 1999 Thomas Adès vied with Blur and the Chemical Brothers. Michael Nyman or Parklife? Let’s toss a coin.

However, a classical release hasn’t shown up for some time, leaving the jazzers as the token minority. But this year’s “jazz” entrant is breaking the rules. First, some sages — including on this paper — have suggested that the Portico Quartet might even win. Secondly, their album, Knee-Deep in the North Sea, is more jazz-influenced than hardcore jazz and has found ready listeners on Radios 1 and 2 among people who would normally run for the door at a Pharoah Sanders solo.

The quartet make good-natured instrumental music, sometimes playful, sometimes rocky, occasionally soporific. Their unique selling point is the hang, a Swiss-invented relation of the steel drum, whose delicate chiming textures add a hint of balmy exoticism — ideal if you want a mental escape from a dank British autumn.

I meet the band at their spiritual home — the South Bank in London — where the four long-time friends began busking in 2005. “We had a spot near the second-hand book stalls with a lot of people passing,” says Jack Wyllie, the saxophonist. “You’re not supposed to play there but the sellers liked us and so did the security people.”

The saucer-shaped hangs fascinated passers-by — Duncan Bellamy had bought his at the Womad festival for £400 and Nick Mulvey soon followed. “We had all played in other bands,” the double-bass player Milo Fitzpatrick says, “and we knew what sort of reaction musicians can get, but with this band it’s been exponential growth. We’d organise student union gigs and pack the place out. People knew all the tunes, all the titles.”

They recorded their own CD. Bellamy volunteered to paint the covers, a decision he came to regret as sales reached 10,000. They hot-wired their laptops in the shared house in Clapham, churning out copies overnight. Two thousand were boxed up and sold on a busking tour of the Continent in the summer of 2006.

Music industry interest was bound to follow — DJs Gilles Peterson and Rob da Bank were fans; the forward-thinking London jazz club the Vortex invited them to launch its new record label. Knee-Deep in the North Sea followed, its name inspired by a night that Mulvey spent at a rave in Norfolk that ended with him wading, trousers rolled up, into the moonlit deep.

The band, still in their fresh-faced early twenties, thought success would be “a slow burn”, but the Mercury nomination has bought sudden media exposure. Album sales are reportedly up 256 per cent — though they have had to reduce the price because all the major labels cut the cost of their shortlisted records — “so we are not making any more money,” Mulvey says. He’s just back from a Radio 1 interview and learning the requirements of soundbite media fast. “I think I mentioned ‘contemporary classical fusion’ and the guy just stared at me. They just want banter.”

Wyllie’s dad and his mates have put £25 each on the band to win. “We were 16 to 1 at one point, but the odds are actually bigger for Estelle and Adele. It’s in the nature of the awards that they pick someone quite left-field.”

So could they actually win? Talvin Singh triumphed with his highly personal mix of East and West in 1999. And rumours persist that performance on the night can make a difference. There is a story that an incendiary showing by Joanna MacGregor, the jazz and contemporary pianist, moved her from rank outsider to a point short of victory in 2002.

But no one is ordering the Cristal yet. Kerstan Mackness, their manager, says: “They have a better chance than almost any other ‘jazz’ act chosen in the past few years but just being chosen as an album of the year is like winning for them.”

In the long term it may be that the Portico Quartet’s real success comes on the Continent, where labels such as ACT and ECM, and audiences, too, are less hung up on the strict division of jazz, pop and classical — and where groups such as the late Esbjörn Svensson’s can become major draws rather than just cult favourites.

For all the exposure, though, the band’s heads remain defiantly unturned. Do they still busk? “Yes,” Wyllie says. “Give us a Saturday when we’re all free and we’ll be out there.”

Knee-Deep in the North Sea is on Vortex

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