Let’s start with the numbers. Not the songs that have made Fleet Foxes the most ardently admired new act around, but the figures that delight their bean-counters: a good-news business story. We don’t need them to prove the heart-melting qualities of the five-piece’s harmony-laden folk-pop, or to explain why their debut album was The Sunday Times’s record of 2008, but they do help to quantify the scale of their achievement so far, and to suggest why an unassuming indie band from Seattle garnered two Brit nominations without really trying.

It’s late February, an hour before they begin the first of three consecutive nights at London’s 3,000-capacity Roundhouse. Backstage, their songwriter and front man, Robin Pecknold (23 tomorrow), is telling me how “rad” it was when he heard that Sub Pop, their home-town label, was shipping 8,000 records to American stores. Fleet Foxes’ album has since gone platinum in Britain (300,000 copies sold), giving Bella Union, which released it here, a new lease of life. In January, it re-entered the chart at No 3.

“That’s insane,” Pecknold says. Fresh-faced behind a bushy beard, he looks like an Amish farmer (albeit one in plaid shirt and jeans) blest with a bumper crop. “I almost don’t have an opinion on it. I don’t have any physical concept of those things. It’s like, if you’d made a really successful toy, people would expect the toymaker to have his mind totally blown. It’s this thing we made, but it’s just a reflection of that time. Whatever happens to it feels like it’s not happening to you, because you’re past that.”

The rapturous response to the Roundhouse gigs should have provided Pecknold with tangible evidence of his band’s success. If he still needs convincing, Fleet Foxes are set to be festival darlings this summer: they appear on the Friday of Glastonbury and the Sunday of Bestival, headline the Saturday at the End of the Road and support Neil Young in Hyde Park. Performances on talk shows and Saturday Night Live have boosted their sales in America, but why has Britain taken the group to its heart?

“It’s hard to say, not knowing the norms of your music scene,” he replies. “But everyone in America thinks we sound like British folk music, and everyone here thinks we’re super-American.” Pecknold is too polite to admit it, but there’s no great secret to their appeal: it’s simply the unique alchemy of wonderful tunes and voices that gel gloriously. A country with as many choirs as ours was bound to lap it up.

“I felt harmonising would be a good way to, like, validate a band or something,” Pecknold explains. “I wanted the full-band songs to be things you couldn’t play by yourself and expect to get anywhere near what the full band was doing. White Winter Hymnal (rereleased as the next single) played solo would be a totally pointless exercise. That song couldn’t exist, or have the same effect, without the full-band treatment.”

While the Beach Boys, Crosby, Stills & Nash, and Midlake are the most obvious American reference points, Pecknold is just as familiar with rather more recondite British source material, from the relatively well-known Bert Jansch to the solo work of Steeleye Span’s Tim Hart and the all-but-forgotten Duncan Browne. With its image from a Bruegel painting, the Fleet Foxes album sleeve nods to a notionally medieval troubadour past — so is it right, then, to detect a certain olde-worlde courtliness in those lyrics about meadowlarks and trysts “at the top of Beringer Hill”?

“Absolutely,” Pecknold says. “My brother and sister and I visited England, Ireland, Scotland and Norway in 2007. We just kinda walked around a bunch and stayed in hostels. It was a ramble. Tiger Mountain Peasant Song and Oliver James were written over here. I guess that stuff just comes out when you’re wandering from stone circle to stone circle. . . ”

That quip is so dry, it could be etched in granite, and Pecknold says it so impassively, his hands clasped neatly in his lap, that he can’t be entirely in jest. Later, his stage banter suggests he’s a completely switched-on guy who can take a joke. When the Roundhouse crowd make cracks about Sonic Youth, Pecknold drawls “Man, you’re at the wrong show” in riposte, and he laughs with the rest of us at the lone request for “something from High School Musical” (not so far-fetched, actually: the preteen Pecknold’s nickname was “Showtunes”, due to his love of Rodgers and Hammerstein). In conversation, however, there’s a shy intensity to him, a steeliness about his artistic aims. He has the air of someone who operates in his own personal time zone. When his guitarist and oldest friend, Skyler Skjelset, mentions that there have been tours when Pecknold has only come out of his hotel room to socialise “once or twice”, you’d bet your life savings that’s because he’ll have been practising quietly, not partying hard.

Pecknold and Skjelset, both of Norwegian stock (there’s a hint of “yah” to their “yeahs”, like Marge in the film Fargo), have played together since they were 13. And Pecknold’s family is hugely supportive of his music: his dad was in the 1960s Seattle band the Fathoms and builds guitars as a hobby; the band’s manager is his sister, Aja (named after the Steely Dan song), a former rock journalist whose description of Pecknold’s singing as a “wavering, golden longing” hasn’t been bettered; his elder brother, Sean, makes Fleet Foxes’ videos. How is it to have his siblings in the camp?

“It’s awesome,” he says. “Aja’s really good. It’s not a thing where if she wasn’t \, she’d be a manager, but I think she’s supremely qualified. Then it’s like, ‘Who else but Aja?’ Someone else could do the videos, but if Sean’s a film-maker and he can, and it’s gonna be cool, then why not Sean?”

Today’s settled Fleet Foxes line-up is something of a Seattle supergroup. Casey Wescott, 28, is a multi-instrumentalist who was a member of the band Seldom; the bassist, Christian Wargo, 32, releases his own songs as Crystal Skulls; the drummer, Joshua Tillman, 27, has five solo LPs under his belt. In 2006, however, an evolving incarnation released a demo EP and gigged in Seattle. YouTube offers footage of a show at the Crocodile Café in May that year, where the band has a jangly, indie-pop sound and the lead vocals are largely unaccompanied. During sessions in Pecknold’s parents’ basement, Wescott’s house and a rented studio with the producer Phil Ek, a family friend, old songs were canned, the lyrics of new ones became more allusive and those magical harmonies emerged.

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