One of pop's great delights is its unpredictability: a month ago, who could have foreseen an unknown Cambridge art-rock quartet landing in the top 20 with their debut album? From the little speech made by singer Joe Newman at this show, not even Alt-J themselves expected it. "Thanks to everyone who bought it – we wouldn't be anywhere without you," he says, close to tears. He's not kidding – this genre-defying blend of plainsong, dubstep rhythms and acoustic folk, played to a rapt sold-out crowd, testifies to the power of word of mouth, rather than record-label marketing spend.

Understandably, there's a culty feel to the show. Though Alt-J have magnanimously claimed the album, An Awesome Wave, to be "quite accessible for an older crowd", the audience is uniformly young, and the fans know every song from its first note. Their enthusiasm is not misplaced: every piece of music consists of disparate components seemingly glued together on a whim, from Newman and Gus Unger-Hamilton's a cappella harmonies to Thom Green's hip-hop drums, though it somehow feels right. In another galaxy, the oddly twinkly murder-ballad Breezeblocks, which has the singers squeezing their eyes shut as they cough: "Please don't go, I love you so," would even be a hit single.

Dissolve Me, played live for the first time, makes a good case for Alt-J being serious students of jazz; its whorls and glitches are so intricate that it's clear the band are influenced by more than just the usual gaggle of angular indie groups.

If there's a disappointment, it's that the set's big moment happens too early. Three songs in, during Tessellate, a choir squeezes through the crowd and, reading the lyrics from sheets of paper, offers an angelic counterpoint to Newman's ghostly lead vocal. For a band just one album old, it's pretty impressive.