Sacha Gervasi was a spotty 16-year-old and the only headbanger at Westminster School when he ran away from home to tour North America with Anvil.

The year was 1983 and the “demigods of Canadian heavy metal” seemed poised for a global breakthrough. It never happened.

A quarter of a century later, Gervasi’s film about the band has rescued them from impoverished obscurity and triggered the major label bidding war that they have dreamt about for 30 years.

Anvil! The Story of Anvil, an irresistible real-life version of This is Spinal Tap, has been one of the hits of The Times BFI 52nd London Film Festival and a host of similar events.

Keanu Reeves introduced the British premiere in Leicester Square on Tuesday night. The audience gave the film, and a postscreening gig by the band, a raucous standing ovation.

Anvil is ostensibly a music documentary but it is really about the extraordinary bond between Steve “Lips” Kudlow (vocals and guitar) and Robb Reiner (drums), childhood friends from Toronto who refuse to admit that stardom has passed them by.

Stuck in deadend jobs and facing disillusioned families, crippling mortgages and the prospect of old age, Kudlow, Reiner and their newer recruits Glenn Five (bass) and Ivan Hurd (rhythm guitar) embark on a disastrous European tour that reaches its nadir in front of an audience of a few dozen at a pub in Prague.

After a fight with the promoter, who has attempted to pay them in goulash, Reiner is asked why they are no longer playing to crowds of thousands. “I can answer that in one word,” he says. “Two words . . . three words: we haven’t got good management.”

Great things were predicted for Anvil when their 1982 album Metal on Metalcame out. But while many of the bands that they influenced, such as Anthrax and Metallica, went on to conquer the charts, Anvil floundered.

Even Gervasi, the precocious public schoolboy who had befriended them at their first British gig in 1982 and toured North America as their roadie the following year, gave up and started listening to David Bowie instead.

He lost touch with his bedraggled “older brothers” for 20 years. He went to university, cracked Hollywood as a scriptwriter (writing the Steven Spielberg film The Terminal) and fathered Geri Halliwell’s daughter.

Then, on a whim, he made contact again in 2005. “I was feeling wistful for my youth so I looked them up and discovered that they had just kept on going. I got an e-mail back from Lips almost immediately saying, ‘Hey Tea Bag (that was what they used to call me), we thought you’d died or become a lawyer’.”

Within eight weeks Gervasi had started work on the film. On day two of shooting, a cameraman locked him in his hotel room and refused to let him out until he told him whether the band were, in fact, actors.

It is easy to see why. Anvil have a song about the Spanish Inquisition called Thumb Hang (“Thumbs Will Twist/Can You Resist?/Beware the names on the Inquisitor’s List!”) and the subjects that Reiner paints in his spare time include an imagined public park with a monumental anvil in the middle of it.

“The thing is, you could not make this stuff up in a million years,” Gervasi said. “They just are very funny.

“The issue has not been the music. It’s the marketing. They are two guys from Canada who are now in their mid-fifties with thinning hair and power paunches. It’s not very sexy.”

Yet finally, after years of waiting and with tongue more than slightly in its cheek, the world may be ready for Anvil: The Comeback.

Anvil will be in cinemas in the spring.

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