A JANITOR ambles on stage at the Royal Festival Hall and contemplates his broom. He begins to sweep - slow, deliberate lunges at the dust - and then, as if it's a novel experience, begins to listen. So does the audience, suddenly aware of the minutest contact of bristle on board. Swish, swishety, thwap, swishety. Another broom- wielding janitor slouches into view and sets to with more gusto. We have counterpoint. Six more appear, then more, until we have a symphony of timpani, complete with theme and variations, riffs, solos, a turbo-driven beat and extraordinary dynamic range. At the climax, with 30 broom handles and 60 workboots drumming con fuoco, the show comes into its own. There is no other word for it but Stomp.

But wait, the stage is dark. Someone clicks on a cigarette lighter. A tinny click, tiny, delicate (our ears are attuning to niceties of timbre), and another, and another. Snap ... snap ... snap. The lid extinguishing the flame has a weightier sound. A two-tone rhythm sets up, clicks and snaps, gets into a groove, Steve Reich-like, changing imperceptibly, all the while accompanied by a fairy light-show. There is a long, breath-holding moment of delicate magic that's there, and gone. Snuffed out like a light.

If the show has any message at all, it is that music is within and without us every second of the day, like a genie ready to pop out of a bottle. No need for tuition or equipment, just curiosity. The joy of Stomp, conceived by Luke Cresswell and Steve McNicholas for their group Yes/No People, is that it hangs on this single, simple idea. It's not new - people were thrumming washboards and spoons 60 years ago - but these two have succeeded in adapting what began as an oddball Fringe show (at the Brigh- ton Festival in 1991) to a two-hour spectacle capable of filling London's biggest concert hall for a week.

The stage set is impressive: an entire wall of junk. Bin lids, radiators, road signs, oil cans, lead piping, corrugated iron, find themselves battened to the vertical from floor to ceiling. Tantalisingly, none of this comes into play for an hour or more. Instead, we get what's missing: the kitchen sink. Six men, in pink rubber gloves and pinnies, appear with steel sink-units slung around their necks on chains, and proceed to "play" them with dishmops. Soap suds fly in a routine that combines both rhythmic and comic virtuosity, ending, inevitably, in bathos, as each washer-upper pulls the plug and empties his waste over a bucket.

Such moments constantly threaten to tip the show into vulgarity, and would, if there were a hint of sloppy execution. But the music never falters. Dancewise, Stomp promised much but delivers little, unless you count stage- managing 30 large metal dustbins, or manipulating a stage- full of packing cases into a shuffling formation dance, or playing gladiatorial pat-a-cake with bin lids. There is certainly plenty to look at. Strangest of all (and trickiest to bring off) is the sight of an entire cast deporting themselves with a perpetual, insolent slouch while thwacking out compound tattoos with un- impeachable accuracy and vigour.

Rumour has it that Stomp is looking for a run in a West End theatre. Let's hope it's soon, and that they build in some matinees. Noisy boys and girls would love it.

'Stomp': Royal Festival Hall, SE1 (0171 960 4242), tonight.