Tearing modesty a new asshole, Tenacious D describe themselves as 'the world's greatest rock band'. The D's two members, Jack Black (vocals and rhythm guitar) and Kyle Gass (lead guitar and backing vocals), name check among their inspirations Kiss, Dio, Led Zep, Hendrix, The Who, Bach, Mozart, Satan and Meat Loaf ('for his theatricality'). Having subsequently reeled off the names of most of the rock dinosaur pantheon, Black finally says with weary resignation, 'We have been called the heavy metal Simon and Garfunkel.

'But it's not parody,' he adds, underlining that hasty qualification with one of his signature eyebrow-slicing manic stares. 'We don't rip off other people's songs and put new funny lyrics to them. We have our own stuff.'

'We try to write the best songs ever,' says the mellower Gass, not entirely helpfully, 'and they come out kind of funny …'

Tenacious D occupy a unique place in rock music. Black and Gass might be comedy actors who have become comedy musicians, but this duo is no Spinal Tap. While they do flirt with absurdity (on the love ballad 'Fuck Her Gently', for example), The D also take their music making very seriously. Thus, Black and Gass are visiting London not just to promote their film, Tenacious D in The Pick of Destiny, but also to publicise their second album and to set off on a UK-wide tour. The film, which features cameos from Dave Grohl, Ronnie James Dio and Meat Loaf, tells the story of the band's origins.

'We've never been in an interview where someone hasn't asked how we got together,' says Gass, 'so we thought if we put it in the movie, it would answer the question altogether.'

'We start before there was a D,' says Black. 'You'll see us form the band and go on the journey, the quest, to become the greatest band on earth.'

Black and Gass first met in 1985, during rock's golden age of bouffant hairdos. Back then, however, these two California-born dudes were part of Tim Robbins' experimental theatre troupe, The Actors' Gang. Initially, the pair didn't get along, but before long Black was giving the performance-challenged Gass acting tips and in exchange Gass helped Black graduate from acoustic to electric guitar. They formed The D in 1994, aired their manically performed mix of folk metal and comedy rock on an HBO cable TV series in 1999, and released their eponymous first album in 2001. It reached number 38 in the UK charts.

All of which begs the question: why did it take The D 12 years to make a movie? 'It just had to be perfect,' Black says. 'All the planets had to align. We didn't want to make a stinker. We didn't have a time limit and this was our masterwork, so we took our sweet-ass time about it.'

'We thought we could get away with not writing it,' offers Gass. 'We tried.'

'We hired some top writers,' says Black. 'It was along the lines of Tenacious D Saves the City of Atlantis. We thought it wasn't really our sense of humour. So we had to nose-to-the-grindstone it, and sat down for five years and didn't do any writing. Then in the last three weeks we wrote it with our friend Liam Lynch (who also ended up directing the film).'

So, Black and Gass found scriptwriting a difficult process, then? 'Me and Kyle,' Black says conspiratorially, 'we'll fight a lot about little details. And sometimes I think he's fighting me just because he wants to win.'

'Have you ever tried to have an orgasm without friction?' Gass points out.

'Exactly,' concedes Black.

'All the great masterworks have probably had a little bit of friction,' continues Gass.

'Anyway,' concludes Black, 'we started with a sentence. This was the breakthrough. It just took us a long time to figure out that we just have to tell the true tale, the origin episode.'

Returning to the present and setting the film aside, what we can expect from the Tenacious D concert tour? 'In the past we've been on tour, just me and Kyle with no production whatsoever,' says Black, 'just to hoard as much of the money as possible. This time we're really going all out. We're actually losing money on this because we want to do something for The Fans. It's going to be better than The Wall. It starts off in Kyle's apartment and ends up in hell.'

'And we pick up a band in hell,' Gass adds, 'and we reckon, "Hey, we're in hell, we have the choice of any musicians".'

And who would their fantasy band comprise?

'I'd go Mozart on bass,' Black says, without missing a beat. 'Bach on the clavier.'

'On drums I'm going to go with the first caveman,' Gass says, scratching his bald head.

'You've gotta go Hendrix on the electric,' finishes Black triumphantly.

We're back to where we began, talking about rock dinosaurs. But while Black and Gass might be inspired by dead musicians, there's no sign of Tenacious D becoming extinct. Heavy metal Simon and Garfunkel or not.

Hell's kitchen

As famed for their generous girth as their love of hard rock, Tenacious D are just the band to sample the best in Scottish cuisine. Interview: Henry Northmore

Battered Black Pudding

Jack Black Now this one I'm scared of. I heard black pudding is just basically blood sausage, is there any truth in that?

HN There is a lot of truth in that.

JB That is just fantastic. It reminds me of a chorizo, a Spanish dessert.

Kyle Gass A chorizo, a dessert?!

JB I actually feel stronger after eating that. This could be my Popeye spinach.

KG Oh that is fantastic, it makes me feel all warm and runny inside.

Haggis

JB It looks like bbrrraaaaiiiinnnss and that is not a good sign. (Takes a bite) That is just the best thing I have ever tasted.

KG Wow!

JB It's a potpourri of flavours: nuts, barley, meat, love. I want you to try this Kyle.

KG Mmmn delicious.

KG Way to go Scotland. So far, so good.

JB So far, so golden.

Scotch Pancake

KG It's kind of a cross between a pancake and a muffin.

JB That is soooo good, you don't even need syrup.

KG I'd like a fried egg on top and some maple syrup.

The Scotch Pie

JB Don't tell me what's in it, just let me taste it. Mmmmmn, smooth consistency, it tastes good, it's got a little spice to it. In a word, delicious-o.

KG Uh huh, it reminds me of baked Alaska.

JB Is that a fish? It looks like apple pie but it tastes like home.

Potato Scone

JB Oh that's good. The potato scone tastes good in my mouth - and you can print that.

KG But how does it feel in your stomach?

JB It feels very good in my stomach. I'll tell you the rest tomorrow.

Scone

JBThat's no normal scone.

KG That's the kinda thing you would eat before going into battle against the English; you feel fortified and ready to kill.

Tablet

KG That looks like a piece of parmesan cheese or butter.

JB I'm guessing this is cheese. (Takes a bite) Wow! That is good cheese, it's pure sugar. I could eat a brick of that.

Shortbread

KG It's really good. It's really a tragedy that we don't have many Scotch restaurants in the United States, besides McDonalds.

Irn Bru

JB What's the magic ingredient?

HN Sugar and caffeine.

JB This tastes good in my mouth, but there is no fruits or vegetables. How do you guys ever take a poo?

KG Do you have Weetabix? That might help. There's an American product, Metamucil. Write to your House of Lords and get them to bring it over.

Tenacious D in The Pick of Destiny is on general release from Fri 24 Nov. The album, The Pick of Destiny, is out now on Columbia. Tenacious D play the SECC, Glasgow on Mon 11 Dec.

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