SCOUTING for Girls are Britain's biggest selling band of the year. Their anthems follow a tried and tested indie pop formula of repeating lines ad infinitum – 'She's So Lovely' being the most heinous example – until they are imprinted on your mind until death, or at least until you turn off Radio 1. This maddeningly catchy piano pop is about girls, growing pains, and wanting to be James Bond (because he gets to kiss all the girls). They are three cheeky chappies from west London, so terrifyingly upbeat they make Belle and Sebastian look like the Smiths. Their name is a spin on the first manual published for Scouts (Scouting for Boys), and their live shows resemble huge, hearty singsongs around the campfire. They call their fanbase the Wolfcub Club. I fully expect to hate them.

Meeting them in Newcastle ahead of yet another sold-out gig, I'm braced for bad jokes (they rhyme 'pretty' with 'fitty' after all), and bad hygiene (they've spent the last year gigging endlessly and will therefore smell like a tour bus). Instead, I find them funny and fresh. They're staying in hotels on this tour because buses are "rank" and though they do act like big kids, this turns out to be part of their charm.

"That's what I like about this band," insists frontman Roy Stride, an ex-Carphone Warehouse employee who writes and sings the songs, five of which on their double platinum debut have made Top 40 singles this year. "Our music is a fair reflection of who we are. If we were at a party, the people in the cool bands would be sitting in the corner, not talking to anyone. We'd be dancing around, chatting to everyone and making drunken fools of ourselves by the end of the night. But the next day everyone would be saying, 'They were a laugh, we'd like to hang out with them again.' That's what Scouting for Girls is about."

Stride admits that at heart they haven't grown up, but that's why they are in a band. "Every musician I've met is a big kid," he says. "What do we have to do? A 20-minute soundcheck and then a gig? All our food and drink waiting here for us? You have to work hard to keep a grip on reality because you can see how people spiral into drink, drugs or becoming a useless idiot." At this point bassist Greg Churchouse chips in. "Or cheese," he says, looking wistfully at their dairy-free rider.

This is unexpected. In fact, since Scouting for Girls went from playing to 150 people in the rain at Glastonbury last year to returning this time round to 50,000, they've had to change their priorities. "I've joined a gym," says Churchouse, shaking his head in disbelief. "We never go on stage drunk," confirms Stride. "There's no way. I have a hard enough job remembering the words as it is." Their rider contains a pyramid of baguettes, a fridge full of beer (though only two are drunk, one by Stride, the other by me), and enough fruit to keep them on five-a-day for the rest of the tour. "You'll notice there are no snacks," says Stride proudly, the way most musicians would brag about the number of alcohol units consumed. Except they don't do spirits, because their crew always drink them and "go loony". "I am missing the houmous though," grumbles Churchouse.

Scouting for Girls' remarkable success means playing longer sets and charging more for gigs, which means less time for partying. Sort of. They tell me their band phrase is 'having too much fun'. "Yeah, like when your nan says, 'Oh, you're having too much fun,'" explains Stride. I'm not sure how rock'n'roll a band that quotes their nan can be. Scouting for Girls are about as edgy as a round table, but this is precisely why they are so loved. "We do party hard," Stride insists. "You never know how long you're going to have all this."

At Newcastle's Academy, the crowds are snaking around the venue hours before it opens. Scouting for Girls adore their cubs and even used to send them free CDs and homemade membership badges until their record label put a stop to all that unprofitable kindness. "When kids are paying £20 to come and see us, it's a lot of money," says Stride. "I remember what it was like only being able to afford to see a couple of bands a year. And people always come back so we feel we have to give them something more. We put everything into our shows." They have even written a song to thank their fans, which they play live. "We know we're nothing without the people who come and see us. We don't take it for granted. We take having a good time and giving other people a good time really seriously."

It has been one hell of a ride for the trio who have been playing together for a decade, though only as Scouting for Girls since 2005. Stride and the drummer Pete Ellard met at Cub Scouts (where else?) when they were six, while Churchouse is "the new boy" who turned up when they were 11. It all started for them in Scotland. The band got their first headliner at Belladrum earlier this year, which was "awesome", and the year before was even more memorable. Scouting for Girls had yet to release a single but 'She's So Lovely' was getting airplay. While they were playing their tiny tent, a promoter noticed the main stages were dead and then saw where everyone had flocked. "People couldn't get in," recalls Stride. "Then they wouldn't leave and demanded an encore. Our manager said, 'You have to do one – the promoters are here and they'll book you again.' We were like, 'We don't have any more songs.' We had to just play the single again."

They are ready to demo new songs for a second album, and seem confident it will live up to the success of their debut and the run of singles from 'It's Not About You', through 'Elvis Isn't Dead', to current single 'I Wish I Was James Bond' . Expect more piano-led anthems, says Stride, and possibly a couple of ballads too. "With our first album, we were signed on Tuesday, met the producer on Wednesday, and were recording on Monday," he explains. "We didn't have time to think about how the songs would fit together. I want a collection of songs that can all be singles, but which you can listen to as a story."

Their debut album has a bonus track in which they reveal their teenage lust for TV presenter Michaela Strachan. Stride promises there will be more quirky characters on their follow-up and tells me he has written a song about Superman. "What about Philippa Forrester?" asks Churchouse. "No, no, she's not as classic as Michaela," argues Stride. "She's a babe but Michaela is a goddess, a legend." Do they ever get to meet their heroes? "One of the guys who works for our label ran into Roger Moore at the airport, gave him the album and told him, 'Listen to the 10th track because you're on it,'" says Stride. "Roger merely raised an eyebrow."

The last year has been "like winning the lottery and Christmas all rolled into one" for Churchouse, while Ellard looks so shell-shocked he just nods and grins. Stride, the most earnest of the group, simply says this is all he has ever wanted. Their feet remain resolutely rooted to the ground, however. "It's all bollocks at the end of the day," says Stride. "We're just entertainers going out and singing songs to people. Any band that takes themselves seriously is full of crap. It's pop music."

• I Wish I Was James Bond is out now (Epic). Scouting for Girls play Glasgow Academy, Friday and Saturday

• www.scoutingforgirls.co.uk