At nine o’clock, the omens are not great. Sly And The Family Stone are meant to be starting their headlining set right now, and the strict curfew on this inner-city festival is 10.30. On the main stage, though, Chris Stein has decided to add a five-minute guitar solo to “Rapture”, while Debbie Harry looks on with a sort of professional vapidity. Blondie, in all their lumpen, functional weariness, aren’t going to be finished any time soon.

It is, then, nearly 9.50 before Sly And The Family Stone arrive onstage. Actually, it’s just The Family Stone. And if we’re being picky and going to believe the Wikipedia entry, it’s not even The Family Stone, but a tribute band fronted by Stone’s sister, Vet, who until recently were called The Phunk Family Affair.

Of course, we were expecting nothing more. Sly Stone’s recent and highly unlikely return to the stage has provoked a bunch of mildly outraged reviews, as collated on that Wikipedia page. We know the form: the band will play a few of his hits, then Sly himself will wander on for a couple of perfunctory tunes. Maybe the band will carry on for a while, but that’ll be it.

It doesn’t sound like a great deal, but the prospect of seeing this strange and reclusive man, who at least was a genius once, is irresistible to me. Barely sighted onstage in over two decades, Stone is playing just down the road from home in Hackney. However ill-advised it may sometimes be, I can’t resist going to see the old heroes, the reformed bands I give a damn about; it’s almost an urge to collect heritage scalps, I guess.

So anyway, here are the band playing “Dance To The Music”, as you might imagine, and to be honest a lot of the crowd don’t seem to have noticed that the leader is absent. They’re a decent funk showband, a little too fond of taking solos, and fronted by a woman – Vet Stone – who’s a capable singer but who doesn’t have much in the way of presence.

It’s OK: the edge of those old records is missing, of course, that precarious mix of tension and euphoria. As they roll out “Everyday People” and “Hot Fun In The Summertime”, the pleasures are less complicated; those of a party band, not a blazing collective with a revolutionary subtext.

But after about half an hour, just as the rain arrives, so does Sly Stone, and everything changes. He doesn’t look great, to be honest – hunched and swivelled, seemingly dressed like Flavor Flav, though at least the towering blond mohawk that he sported at the 2006 Grammys has gone. But then he starts singing “If You Want Me To Stay”, and it’s terrific; fragile and wandering a little, but with all the emotional resonance that he summoned up nearly 40 years ago. It’s quite moving, too, and thankfully the band have cut the showy fills.

Earlier reports have portrayed Stone as an invalided, Wilsonesque figure, marooned behind his keyboards, but he seems much stronger and stranger than that. Next is “Sing A Simple Song”, that impressionistic, wasted mutter still beguiling, and he’s doing this shambolic dance, a kind of stooped shadowboxing, round the stage. Watching him, it occurs to me that his short appearances are not due to physical infirmity, quite, but more down to an extreme version of soulman narcissism. We will tolerate fleeting appearances, goes the thinking, because we are grateful to be in the presence of such a stellar talent.

The thing is, if you go to a show like this expecting to be disappointed, as I did, you can deal with such primadonna-ish behaviour. After two songs, Stone appears to say something about going for a piss, and disappears. The band look momentarily flummoxed, and the bassist decides to play another solo. But then they just carry on with the set without him, with “Thank You (Falettinme Be Mice Elf Agin)”.

Amazingly, he comes back, and “I Wanna Take You Higher” is great, too. “Stand” is even better, and now Stone is sat on his piano stool, spinning round and round, singing with this great parched feeling, and – judging by the close-ups – an expression of justifiable rapture on his face. Towards the end, he scuttles off without even acknowledging the crowd, and seconds later the curfew shuts down the band. That’s it.

OK, I didn’t pay to get in, so I can’t say for sure whether I would have felt ripped off as a ‘real’ punter. It’s not churlish to expect more than four songs from a festival’s headlining act, though I personally suspected we might only get three. I do feel, rather sheepishly, privileged to have seen a legend in my own backyard, still with a palpable grasp on his genius. He may be back here again and again, deconstructing his myth with revival after revival. He may never play again. Me, I’m just glad I made the effort tonight.