Jamie Cullum, Colston Hall, Bristol

Talking 'bout his g-g-generation? Well, it's an argument...

I know the secret of Jamie Cullum's success: you want to cuddle him, mother him and shag him, all at the same time. There's also a cartoonish combination of frisky physicality and vulnerable, little-boy-lost appeal that is very winning. Bounding on to stage at the beginning of the show, 24-year-old Jamie is jazz-Bambi: long eyelashes flicking, rubber legs running away with him as he does a couple of quick jogging circuits in a kind of animated flirt before grabbing the mic for some stand-alone vocals, as if to say: "Look ma, no piano!" The band - and it is a real band, beefed up to a septet from the piano trio format of earlier shows - has opened with a rather worrying New Orleans funk version of "Lucky So and So", like The Meters meets cruise-ship showband. "Oh no!" you think, "Jamie's gone rock", and remember the time Harry Connick Jr (I won't mention him again, all right?) played the Royal Albert Hall with his new heavy-funk group and lots of older Sinatra fans walked out.

Then, after a couple of verses and more manic circuits, Jamie springs over to the baby Yamaha, jumps kittenishly on to the piano stool and stamps on the keys, Little Richard-style.

Point made, he then sits down to play properly and we breathe a sigh of relief. A bit of fun and games is all well and good, but we know what we want from our Jamie: more of the same, please. The lovely "All at Sea", from the double-platinum selling Twentysomething album, follows, and you can feel the sell-out audience - twenty-, thirty- and fortysomethings, mums, dads, grannies, teenies, the most mixed audience I have ever seen - floating in a smoochy haze of satisfaction.

When he goes into "What a Difference a Day Made" sung dead slowly, that cute, adenoidal burr in his voice caressing each syllable, the smooch-alert hits meltdown levels. As the Wiltshire-lad tells us, it's a home-town gig for him, and that he last came here to see Tony Bennett with his gran, we go collectively misty-eyed. Diddums! Of course, we jazz critics can't bear Jamie Cullum, partly because he's popular but mainly because he's crap.

It's OK, I'm only joking. The real reason we can't bear him is because he's actually really good. If he was crap, it would be so much simpler; we could just write him off as a poor man's Harry Connick Jr (oops!) and have done with it.

But Jamie Cullum has an undeniable spark to him that these days is sufficient to pass as the flame of star quality. Having Universal Records, TV ads, Michael Parkinson, Radio 2, and now Radio 1, too, on his side can't hurt either.

Sure, the reasons for Cullum's success might have more to do with sociology than they do with jazz, but you could say that about anyone. We can't deny the general trend towards niceness in Noughties music: Dido, Norah, Katie, whiny old David Gray. Although it's possible to detect a sea-change in the success of bad girl Amy Winehouse, who isn't slow to get her claws out, there seems to be an almost unquenchable demand for music that a) sounds good with soft furnishings, and b) doesn't frighten the horses, women or children. Property ladder music you could call it. And that, ultimately, is the trouble with Jamie Cullum, or at least it is if you're nasty like me: he's just that little bit too nice. Even the arrangements of songs by members of the band stick to a polite mid-range of musical ambition, with the different reference points - the funk, the smooch, the punchy, big-band-style horn stabs - delivered as if in musical inverted commas. It's the only show I've ever been to where the (excellent) drum solo came as a welcome relief.

A terrific version of The Neptunes' "Frontin'" was enough to suggest that Jamie might profitably play the music of his own generation, rather than The Who's "My Generation", which sounded rather lame. Getting the wrecking-crew rhythm section of Philadelphia's The Roots to back him on his next album, or the trumpeter Roy Hargrove, would be great, too. But it won't happen.

Interestingly, it's still in the pared-down trio format that Cullum sounds best. Some aspects of the new, bigged-up show came across as rather corny, but Cullum has such an engaging presence - and such adoring fans - that at the moment he can carry almost anything off. Strapping on an acoustic guitar to play a new song about the pleasures of procrastination, he showed off not only a neat way with a lyric, but considerable Coldplay-potential. That dreaded phrase, the all-round entertainer, comes to mind.

But what does jar about the new Jamie, and the new show, is that what we've bought into up to now is the smoochy, intimate, piano trio, jazz club vibe, and the larger gestures and coarsened sound quality of a big hall don't sustain this.

We want Jamie to sing just to us, but it's hard to ignore the other 2,000 people playing gooseberry, even in the dark. So where does Jamie Cullum go from here, apart from arenas? I know: let's ask Harry Connick Jr.

Jamie Cullum: Shepherds Bush Empire, London W12 (0870 771 2000), Tuesday and Wednesday