In a career made up of questioning assumptions, upending odds and defying convention with varying degrees of spectacle, it should come as no surprise that Public Enemy continue to confound expectations. So there is a strange sort of logic to seeing them performing for a monied elite, a bare hundred people willing to shell out upwards of 250 quid each to see the band playing a gig in a space smaller than many London flats.

It's at once ridiculous and yet entirely fitting: the band who've spent more than a quarter of a century fighting the power, slotting in a gig where the sense of occasion is an inverse function of the number of tickets available. After all, this is the new way: now that the mass audiences of the last century by and large refuse to pay for recorded music, and the live arena is where bands have to earn their corn, what's wrong with maximising the income available from proximity - one of the few commodities that are still scarce enough to command top dollar?

Other acts make millions playing gigs for sheiks' daughters or private functions for corporations: it's hard to begrudge even as for-the-people a band as PE if they want to spend a rare night off on their umpteenth world tour playing to a tiny crowd willing to pay through the nose for the privilege of breathing the same fetid air as Chuck, Flav and the boys in the band. Besides which: there are solid creative benefits to be found if you approach the occasion with a bit of imagination, and that's never been something that this lot have lacked.

So Chuck D's approach tonight is to send up the occasion and milk it for all the artistic juice that can be wrung out of it. Over the course of a good 20-odd gigs in the past quarter-century, this is by some distance the most relaxed and informal PE show your correspondent has seen: and as Chuck jokes about how much people have paid, and ribs the half-dozen super-special-VIPs who are in what amounts to a large fish-tank in an upstairs room, watching the gig from a glassed-in eyrie, he and his group tackle their dazzling back catalogue with a kind of joyous freedom that's almost knockabout, always rewarding, and frequently also very funny indeed.

PE have always taken touring far more seriously than most rap groups, and they've never stopped learning. They've drunk deep from the same wellspring The Roots discovered, and for some years have managed to seamlessly blend samples and live grooves. In the 1980s, there were two PEs: the studio team and the live line-up. The latter were created to play the former's music in public, but the tail soon began to wag the dog, with the studio squad creating faster, more intense music to feed to a ravenous live audience who'd heard nothing like it before. Today, the live Public Enemy is a different beast entirely from the recording outfit, with the original studio creations merely the starting point for exploratory journeys into sound. Freed from the constraints that a "proper" gig possibly imposes, those voyages of discovery range a good bit farther and wider across the band's musical map than often tends to be the norm.

So tonight the Isaac Hayes piano sample that makes 'Black Steel In The Hour Of Chaos' so hypnotic on record is merely a signpost towards the song it used to be. Khari Wynn's guitar takes the lead as the group mines a blues-funk seam, the groove enormous and the anger made manifest where brittle paranoia stalks the original. 'Fight The Power' morphs from the propulsive loop from 'Hot Pants Road' into a different J.B.'s track - 'Soul Power' - before taking a detour into another Fear Of A Black Planet track, 'Who Stole The Soul?', then coming back around to '...Power's final verse. It may have begun as a musical version of wordplay - in the same way the original song was a nod to the Isley Brothers cut it took its name from - but it ends as a cross between a musicological excavation of various different pasts, and a thrilling demonstration of the all-too-often unrealised power latent in songs that sometimes can seem blunted by over-familiarity.

There are points where the bonhomie and laid-back vibe gets the better of them. Flavor's increasingly lengthy spots demonstrating an advanced but perhaps less than virtuosic capability on a wide range of instruments have long been part and parcel of the 21st century PE experience, but here they interfere with the set list to at first frustrating, but ultimately fascinating effect. Purloining Davy DMX's bass early in the show, Flav refuses to give it back, playing a noodling funk riff even as Chuck completes a lengthy spoken introduction to 2012 album track 'Hoover Music'. Undeterred, Chuck goes with the flow, rapping the lyrics to the 27-year-old single 'You're Gonna Get Yours' as Flav locks in with drums, turntables and guitar, funk morphing with blues to become a slouching, simmering kind of groove that has feet in soul, jazz, rock and rap. It's a telling moment: improvised order out of self-manufactured chaos - the Public Enemy story in sharp-focus microcosm. And quite possibly worth paying a quarter of a thousand pounds for on its own.