Spinal Tap, Wembley Arena, LondonTake That, Old Trafford Cricket Ground, Manchester

Somehow a fictional band of metalheads can still attract tens of thousands almost 25 years later

There can't be a rock band anywhere who haven't had a copy of This Is Spinal Tap on the tour bus. Not only is Rob Reiner's tale of a gormless heavy metal act neck-and-neck with Airplane! as the funniest film ever made. It's also so uncannily true to life that, ever since its 1985 release, actual rock dinosaurs have been squabbling over who is the real-life inspiration.

A quarter of a century on, Spinal Tap have arguably been superseded by the likes of Anvil, Steel Panther and The Flight of the Conchords, whose tele-vision series is a painfully funny expression of what it's like to be in a struggling band now.

The problem with writing a review of Spinal Tap live, a document – or, if you will, rockument – of a concert by a band who don't actually exist, is whether to treat it as a gig or a fan convention. The danger is that, when stripped of the comedy context, you're simply left with a third-rate band, and not a lot of laughs.

Most of Wembley is here to worship the Tap just for being the Tap. Bassist Derek Smalls (Harry Shearer), brilliantly, looks exactly like Derek Smalls would in 2009: identical to his younger self, but with grey hair and 'tache, and leather slacks instead of skin-tight nutcrushers. Similarly, Nigel Tufnel (Christopher Guest), the preening axeman who shows his versatility by parping tunelessly on an Alpenhorn, is thicker set but rockin', with the same look he always had, and David St Hubbins (Michael McKean, left) is still giving it the full Parfitt.

They're assisted by keyboard player Caucasian Jerry Vanston, death-defying drummer Gregg Bissonette and a backing singer dressed like an Australian's nightmare, and on purely musical merits they're surprisingly proficient. Playing the riffs from "Sunshine of Your Love", "Daytripper" and "Jump" all at the same time is a neat trick if you can pull it off. On "Big Bottom", they're even joined by some proper musicians: ELP's Keith Emerson on keys, The Sweet's Andy Scott and Justin Hawkins from The Darkness on bass.

The set spans all eras, from The Thamesmen's faux-Fab Four "Cry All The Way Home" through the weedy psychedelic of "Listen to the Flower People" and cock-rock classics such as "Tonight I'm Gonna Rock You Tonight" right up to new tracks like the brilliant "Saucy Jack" and climate change number "Warmer Than Hell" ("Satan went to Surrey, sweating like a pig ..."). "We premiered that at Live Earth," says Smalls, "and in all modesty we have to say it worked. Two years on, the Earth is still here."

For all the humour in the lyrics, it's the dialogue that we're here for. Like David and Nigel reminiscing about their native district of Squatney: "The whole area's gone now ... there's just a giant poster for Jersey Boys."

The funniest moment clearly was not scripted. Just as it was in the film, the song "Stonehenge" is scuppered by a prop malfunction. This time, a giant trilithon fails to inflate, and has to be wrestled erect by a team of dancing dwarf druids. "We never did get that right, did we?" sighs St Hubbins. "Worth every cent of the £10,000 ..."

A sum which is dwarfed, if you'll pardon the pun, by what it must have cost to put on the current Take That tour. Before the man-band, whose phenomenally successful second career has entered uncharted territory for a revived act, have even appeared, we're entertained by old-fashioned clowns. Suddenly, the performers huddle in a cluster of helium balloons, release the ribbons, and there they are: the reassuringly homely Gary Barlow, the impish Mark Owen, and the two pieces of stubbled mature beefcake, Howard Donald and Jason Orange, whom no one can be bothered to tell apart.

Old Trafford erupts, and so does the outlying area. The estates all around the stadium have thrown impromptu street parties, with fans perched on walls and peering through gaps.

For the first half hour, Take That's Circus tour is about as feel-good as you can get. It's essentially Britain's biggest hen party, but when I spot a green-haired punk in a Rancid T-shirt going nuts to "Pray", I realise just how far the foursome's appeal has spread.

There's a slump when they seek to prove they're a "real" band who can play their instruments, but once they ditch that idea we're back on course. It's a show that tracks their musical development from the oiled-up gay disco of the Nineties ("Relight My Fire") to the stompalong pop of the Noughties ("Shine") via their one moment of towering greatness, "Back for Good".

What they've gained, since their hiatus, is a redeeming sense of their own ridiculousness, plus a licence for pure smut. "Go on, Jay," says Barlow, as Orange strips to his boxers. "Play your one-note skin flute ..."

The hands-in-the-air hosannas that greet the chorus of "Never Forget" come as a shock if you didn't know which song it was, and are almost as spectacular as the pyros that mimic musical notes in mid-air.

This is what's known as a home win. Robbie who?