"I said 'Fuck you' to the record companies," begins Anvil's frontman, Steve "Lips" Kudlow. "My asshole is still hurting!" Then he whips out a metal vibrator to play guitar, tells us war is wrong because "we should all be friends", and piles into School Love, an eyewatering racket involving corporal punishment and naughty schoolgirls.

Ever since the documentary Anvil! The Story of Anvil chronicled a career of empty gigs and nights spent on airport floors, Canada's veteran flop heavy metal band have been dubbed "the real-life Spinal Tap". Ironically, the film is making them almost successful, with a cult following developing.

Their music might be a ghastly rumble, but their end-of-tour japes provide more opportunities for Spinal Tap comparisons. During 13 (a song from This Is Thirteen, their 13th album), someone behind them holds up a sign reading "31" and then, even more bafflingly, "12"; meanwhile, in the middle of Metal On Metal, a tiny polystyrene anvil descends towards the stage.

Saxon, who reputedly inspired much of the original This Is Spinal Tap movie, followed – and the Barnsley rockers' granite-hard songs about slaying beasts and riding motorcycles still command a huge audience. Fronted by Biff Byford (still long-haired at almost 60), the band at the forefront of the New Wave of British Heavy Metal haven't changed their sound in 30 years (and "several wives").

However, while watching Byford might feel like a privileged glimpse of a living brontosaurus, he acknowledges that a song called Crusader, about medieval religious wars, might get them "into trouble" nowadays. Not that this stops him. "Take us to the 12th century, Mr Quinn," he instructs the guitarist. And, for a few minutes, it sounds as if these kings of old-school metal really have been around that long.