No matter how witty they may be, few satires or novelty songs repay repeated listening. So how come a packed Wembley is rocking to the soundtrack of a mockumentary that appeared a quarter of a century ago?

Rob Reiner's cult 1984 movie This Is Spinal Tap launched the hapless spoof British metal band made up of singer David St Hubbins (Michael McKean), guitarist Nigel Tufnel (Christopher Guest) and bassist Derek Smalls (Harry Shearer). Twenty-five years on, the joke still works, both because their increasing years and spreading waistlines add to the air of out-of-their-time befuddlement, and because the rock dinosaurs they mocked – Def Leppard, Whitesnake, Saxon – are still trundling around the circuit.

Tap's latest album, Back from the Dead, is largely a cash-in, but this self- billed One Night Only World Tour show is joyous because of their assiduous attention to detail. Their parodies of both 1970s prog and heavy metal cock-rock remain painfully accurate, yet lewd 1984 anthem Sex Farm has been transformed into a funk workout with a preposterously clunky rap break. "We've urbanised it," explains St Hubbins, helpfully.

Smalls lauds the success of Warmer Than Hell, the climate-change song they wrote for Live 8 ("The Earth is still here! It worked!"), then two dwarf druids dance around a rapidly inflating and deflating henge for the infamous pseudo-mystical Stonehenge. For the climactic Big Bottom, a smug St Hubbins surveys an on-stage scene involving scores of jiggling female dancers, 1970s keyboardist Keith Emerson, ex-Darkness singer Justin Hawkins and somebody from the Sweet, and finds the words that he needs to say: "Thank you, Wimbledon!"

And with that, the rock gods are gone.