Whoever misrepresented the importance of Jamie Cullum's place in jazz evolution, it was never the man himself. The 26-year-old singer/pianist from Wiltshire plays every show as if nobody's more grateful and amazed to be there than he is, and that bounding enthusiasm has played a big part in his popularity.

This sold-out week at Ronnie Scott's launches a tour and the new album Catching Tales - following 2003's Twentysomething, which is reputedly "the fastest-selling jazz album in UK history". Promisingly for his musical future, Cullum doesn't buy his own publicity, or believe that he's in the Herbie Hancock class as a jazz virtuoso with a pop knack. Cullum knows he's a maverick pop artist who has picked up attractive fragments of jazz along the way.

He ran down his hitlist early in the evening, but for his midnight set (following a deft balance of live-loops flute music, and imaginative fusion and postbop tenor-sax grooving from Theo Travis's band) he left things loose. These Are the Days came in at his familiar gale-force momentum (with guitarist Sam Wedgewood also supplying trumpet and organ licks), and Doves' Catch the Sun balanced the singer's fresh and unfussy handling of a pop lyric with a fierce funk hook powered by that fine drummer Sebastian De Krom. The languorous blues of Back to the Ground, and the ironic Seven Days to Change Your Life (inspired by an American TV-makeover programme) opened with some smoky small-hours piano rumination, of the kind that confirms Cullum's genuine affection for orthodox jazz.

But, as with his early appearances at Pizza Express a couple of years ago, he was at his most expressive on the simplest of materials. A duet with elegant double-bassist Geoff Gascoyne on the Nat King Cole classic Nature Boy turned into a nimble scat over a bass riff that became the Twentysomething hook, and I Loves You Porgy brought some delicate Jarrett genuflection on piano.

Then Cullum was on his feet and bouncing again, playing an encore on Singing in the Rain as a funk blast. Whatever is said about him by others, Cullum looks as if he just wants a good time playing, and it's an infectious ambition - maybe because he doesn't seem all that far ahead of an audience that would like to be doing the same.

· At Ronnie Scott's until Saturday. Box office: 020-7439 0747.