'We're doing this", Jake Shears reveals mid-show, "to get a load of money together to buy drugs..." There's an unintended comedy pause, during which you admire his candour but wonder how this makes Scissor Sisters different from any other band on the planet, before he continues "...to fight Aids in Africa."
Trafalgar Square is swathed in red. The stage in the shape of an ornate gilded picture frame (a gag on the Gallery behind them), the newly-unsheathed Nelson's Column turning his kiss-me-Hardy butt toward the band, and 5,000 Londoners who won free tickets, are all bathed in scarlet light. Before a note has been played, there is already a carnival atmosphere, and it looks way cool. This is the doing of the "(RED)" initiative, a scheme founded by Bono (who else?) and Bobby Shriver (who?), which persuades leading companies to give 5 per cent of the profits from certain products to humanitarian causes. Shears's trust in the altruism and sincerity of the multinationals is perhaps naive, and the prominent logo of a certain monger of mobile telephony tells you what this is really all about, but it's touching, and his heart is in the right place.
Scissor Sisters, of course, have a heart. This immediately makes them unusual. Furthermore - not a band to leave the Emerald City empty-handed - they have a brain. This makes them virtually unique. And it's been a joy to watch the rise and rise of this band you can believe in. This is how you make a pop group. None of that oppositional "alternative" nonsense, no fake anger, no focus-grouped designer doom. Just a bunch of multicoloured misfits, making 1970s radio pop and euphoric disco with a spirit of devil-may-care, live-for-the-moment fun, and travelling the world preaching diversity, tolerance and the pursuit of a brilliant night out.
"A good time is good," says Ana Matronic, whose pronouncements err just on the right side of cheesy sentimentality, "but a great time? That's therapy. And the best therapy of all is dancing." She is the wisecracking elder sister you wish you had - half Dorothy Parker, half Rizzo from Grease - though she's almost upstaged as Queen of the Square by Kylie Minogue who introduces the show, making her second public outing since being treated for breast cancer.
Ana and Jake are a consummate comedy duo. She tells him (accurately) that he looks like "a matador moonlighting with Siegfried and Roy". He says he's delighted by the story that Prince William has attended a fancy dress party as a Bond girl: "It's good to see the future king of England has got a bit of queen in him." They both flatter the crowd by calling London "our new home town", and postulating that the reason for Scissor Sisters' popularity in Britain relative to their anonymity back home is that "Brits, on average, are smarter than Americans".
The songs, though, outshine even the personalities. Opening with "Take Your Mama Out", and alternating between tracks from their debut and their latest, Ta-Dah, it's a show which already - from a band only two albums into their career - feels like a greatest hits set.
"Laura" gets the loudest singalong - 5,000 voices shouting "shamone!" back at them - and, of the new tracks, still largely unknown to this audience, "Kiss You Off" (Amii Stewart's "Knock On Wood" meets Blondie's "Call Me") stands out. By the time we reach "Comfortably Numb", the security guards have been overwhelmed and the square's two fountains, their water jets lit blood-red, have been invaded by bare-chested, boxer-clad revellers waving feather boas, like a gay Noughties parody of the Scottish football hooligan invasions of the 1970s. It's a glorious sight. After an encore of "I Don't Feel Like Dancin'" - a strong contender for Single of the Year - the Scissors stage an invasion of their own, with two swordfighting Lord Nelsons and two dancers in padded "thalidomide victim" body suits (a reference to the sculpture of Alison Lapper on the square's spare plinth) for a finale of "Filthy/Gorgeous". No other band could get away with this, but then, no other band could have given London such an unforgettable night.
That said, a few words in praise of dumbness: "Yeah motherfucker, say it!" yells the man jogging on the spot in the questionable purple tracksuit, who has clearly picked up more from living in Los Angeles than a strange Bradford-goes-to-Hollywood accent, "This is Cult music comin' at you!"
Ian Astbury, most recently seen channelling Jim Morrison in the 21st-Century Doors, has long been one of rock's most irony-challenged savants. If he wasn't, The Cult simply wouldn't work. He genuinely believeshe's a wolfchild, and recently spoke in an interview of the awakening he felt while howling with the white wolves in Berlin zoo. "Wolfmother, Wolf Parade, Wolf Eyes," he sneers, reclaiming his lupine lebensraum, "I don't think so. These young bands, do they even know how to rock? That's why we're back."
The Cult struck gold in the mid-Eighties when they reinvented themselves as a kickass boogie band in the style of AC/DC - they were The Darkness before The Darkness were - and it's that period which dominates this latest comeback. "Lil' Devil", "Love Removal Machine", "Wildflower" and "Fire Woman" (about whose thrilling intro alone I could write pages) prove that, for a while back there, Billy Duffy was rock's leading riff machine.