Music: Biffy Clyro

IN A Masonic temple in London's East End, a hirsute, tattooed Scotsman sits on a throne. Flanked by guards, he is dressed head to toe in flowing black robes with a breastplate sprouting shaggy tails of human hair – not his own – taped to his chest. I can't see his face all that well, obscured as it is by yet more hair, this time his own. But I don't need to. Who else could this be but Ayrshire's biggest, bushiest rock god: Simon Neil, lead singer of Biffy Clyro.

I'm here to watch the band many are describing as the British Nirvana (or is it the Scottish Foo Fighters?) shoot a video for their forthcoming single, 'Mountains'. It's an anthemic, supersized epic, perfect for rocking out a summer of festivals, and sure to have Team Biffy, the band's rabid cult following, zealously roaring along to the chorus that Neil is currently lip-synching aggressively: "I am the mountain/I am the sea/You can't take that away from me."

Even though the person from the label tells me the band were asked to get shorn for the video, Neil still looks pretty furry to me. "Look angry!" the director encourages, and he obligingly stares through his beetle brows into the camera and mutters: "I'll f***ing kill you!" It's all rather impressive, apart from the fact that his breastplate keeps falling off, and when he gets up I notice the back view isn't quite so rock opera with his skinny, pasty Scottish legs peeping out from his robes. I resist the urge to sound out the Team Biffy battle cry – "'Mon the Biffy!"

The band are into their 19th month of a marathon tour following the phenomenal success of last year's Puzzle, their fourth album. This audacious set of leftfield rock songs is already considered a classic of its kind, and saw Biffy cross over into the mainstream after some 15 years of quietly headbanging away in Scotland. Since then they have supported a who's who of rock legends – the Rolling Stones, The Who, Queens of the Stone Age and the Red Hot Chili Peppers. They have played T in the Park more than any other band, and this is their eighth year. "Twice more than Travis," Neil tells me proudly.

And, despite their penchant for destroying equipment on stage – most recently in New York in protest at their set being cut – they are surprisingly sweet, gentle souls, describing Neil's recent wedding to his childhood sweetheart in Ayr as one of the best days of their lives, and talking about how lucky they are to have each other. Neil even waxes lyrical about his belief in love at first sight, "because it happened to me, and I married her". Forget Travis being the nicest men in pop, Biffy Clyro are the most romantic men in rock.

"This song is about life after Puzzle, and trying to build things up after going through a rough time," says Neil of the forthcoming single when I manage to grab him between pretending to play a pipe organ and overseeing a martial arts fight. "Out of the worst times I could ever possibly have in my life came the most amazing time with the band. And it made me realise I wanted to marry my girlfriend." Neil is talking about the death of his mother while the band were making Puzzle. She was just 52 and his grief inspired tracks such as the Wagnerian 'Living Is A Problem Because Everything Dies', and 'Folding Stars', in which Neil howls his mother's name over and over again in the chorus.

"I'm a glass half empty kind of guy, so I always expect something bad to happen," he continues. "It was a worry putting out a record exposing my own life so much. If people hadn't liked it, it would have been a real knock and I wouldn't have known how to write music after that. If I can't write honestly, I'm up shit creek, so it was a relief that people understood."

Biffy Clyro has Scotland running through it like a stick of rock. Not only do the trio all still live in and around Ayr, they all talk about Scotland constantly, and make me promise I'll mention a special homecoming gig they're planning towards the end of the year to thank their Scottish fans. The day after I meet them, they are returning to Scotland for just 23 hours before heading back to London to support Jon Bon Jovi. "If we'd moved to London, I think we would have ended up losing our way," says Neil. "It's got to matter to us at home because that's real life. This is part of our life, but it's not real." He looks down at his costume and starts laughing.

So far the two other members of the band, James and Ben Johnston – twins from Kilmarnock with hair the colour of Irn-Bru and matching tattoos – haven't arrived. The bassist and drummer respectively, who have known Neil since they were seven, will appear in the performance part of the video filmed later today. The band's manager tells me Biffy rarely rise before lunchtime, so presumably they're in bed.

The entire video has to be shot in one day, but the atmosphere is calm and studious. It's a stunning location, all marble columns, mahogany panelling and Masonic symbolism, and the theme of the video is a kind of ritualistic face-off between the king of the mountain (Neil) and the king of the sea (Neil again), with some chess and martial arts thrown in too. They picked this location because of the black and white marble check floor. Because it's a listed building, they can't rig up any lighting so an enormous submarine-shaped helium balloon hovers above us, lighting the windowless room.

One of the crew tells me the band discovered this place at a party a fortnight ago. James bursts into laughter when I ask him about it. "Are you joking? We don't go to parties in places like this. It was the video people who found it." Rocking up in jeans and T-shirts with an American Apparel bag stuffed with yet more jeans and T-shirts, James and Ben look distinctly ordinary. When one of the assistants on the shoot tells us not to eat too much because there is only so much food, she doesn't realise she's talking to two members of the band. They don't take it well.

Over the past year or so, the band have been away from home for long stretches. "We miss our girlfriends," says Ben. "We're still very much small town boys. And we even miss the rain, when you're in a place that doesn't have the right kind."

"It has to be sideways and hitting you in the face," confirms James.

They show me their tattoos, a living document of the band's history. "We've all got the Biffy logo, which we got on James and my 21st birthday," says Ben. They also all have a puzzle piece to represent their last album. It's easy to see how they adore their band. Neil even waited four months after getting married to go on his honeymoon so that he could go back on tour.

"We don't have plans to go on and do another band," says James. "We're doing this because we love it."

"We don't have a plan B, and as soon as you do, you're screwed," agrees Ben. "It's never been about being in a band to make money or get famous or meet girls. It was all about writing songs."

One mystery, however, remains. In the past the band have said they took their name from an Ayr United footballer, but they have also said it's about a Cliff Richard pen (Cliffy Biro). Today, I get a different story. "It's an acronym," reveals Ben. "Big Imagination For Feeling Young 'Cos Life Yearns Real Optimism. That's the real answer." They both fall about laughing. What's so funny?

"We've just given away a massive secret," says James between chuckles. Somehow, I don't think this is the end of the story.

Biffy Clyro play T in the Park, July 12. Mountains is out August 25 (14th Floor) www.biffyclyro.com