"When I saw pictures of myself, I realised I was a bit fat. To be a pop idol required a certain kind of body - particularly if I was going to wear jumpsuits," he says in an interview to mark the 40th anniversary of their Eurovision victory and the publication of Abba: The Treasures, a book of unseen memorabilia and personal anecdotes put together by his friend Ingmarie Halling.
He needn't have bothered. Abba, formed in 1971 when best friends and songwriters Benny and Bjorn began dating singers Anni-Frid (Frida) and Agnetha, were never destined to become rock stars in the traditional sense. By 1974 they were seasoned performers, married with two children each. In their 10 years together, Bjorn says they were never once offered drugs.
"We were considered too Scandinavian and too boring," Bjorn says. Yes, they wore those clothes in total sobriety.
Halling, who became a close friend of the group after touring with them as a wardrobe assistant through Europe, and to America and Japan in the mid-70s, attributes the band's success in part to their complete indifference to what was in fashion.
"They never asked anyone else's opinion or tried to copy, and this enabled them to make a mark," she says. Evidence of this is evident throughout her book: a snap of Bjorn in white Lycra dungaree flares, for example, or a shot of the band wrapped up together in silver foil.
Bjorn has no shame in admitting that the show-stopping outfits worn that chilly spring night in Brighton were part of a deliberate attempt to get the band noticed by the British public.
Behind their beaming smiles and tousled mullets was a steely confidence in their talent, coupled with a healthy cynicism about the music industry. All four had enjoyed success in other bands or as soloists and had chart hits with People Need Love, Nina Pretty Ballerina and Ring Ring since coming together as Abba - an acronym of the first letters of the band members' first names.
"When we first went to Los Angeles I could easily see through the bullshit," Bjorn says. "If I'd been 20 I might have believed all that we were promised."
Instead of moving overseas to pursue their career internationally as Abba-fever took hold, they stayed in Stockholm and worked even harder. For Bjorn and Benny, songwriting was a 9-to 5 job. Every day they'd meet at an office they'd rented, or in Benny's basement, while Frida and Agnetha, who'd become a reluctant pin-up girl, stayed home with the children [Frida had two children from a previous marriage, Bjorn and Agnetha had one at that time].
"There would have been so much temptation if we'd been in London or LA," Bjorn says.
"In Stockholm everyone knew us so we weren't a big deal. This meant we could continue working in a very disciplined way."
It helped that they were great friends with young families, which ruled out too much partying. "We were four people each with very strong wills so, as you can imagine, it got really tense sometimes," he says. "But we have democracy in Sweden and we managed to keep that in the group, too."
They applied the same approach to their technicians, designers and musicians. Halling remembers one occasion when Benny stopped a concert midway so the crew could have a hot meal. The production company hadn't thought to supply one and he was furious.
"They knew everyone's names and made them feel welcome," she says. "It's a very Swedish way of working - if someone doesn't feel like they belong, they don't do a good job."
The fans adored them - their 1976 hit Fernando went to No 1 in at least 13 countries - but from the start, music journalists had it in for them. Halling's book contains newspaper cuttings describing their music as "cold-hearted cynicism" and "empty".
"In the 70s it was politically incorrect to listen to Abba, even in Sweden," she says.
The band ignored the critics. They'd dealt with bad reviews before in their individual careers.
"In the beginning everything we did was crap," Bjorn says. "But we knew from experience that there was nothing more important than the song. And you quickly learn to ignore the criticism when you hear reports from all over the globe that you are No 1."
It was only on tour that Abba finally got a chance to behave, to some degree, like the megastars they were. Halling's personal photographs convey the glamour of touring: the fans, the private jets, the concerts and the sunbathing. Among the reservation forms, tickets and itineraries folded into her book is a note from Frida recalling more than 20,000 fans waiting at Melbourne airport. "First, you couldn't believe your eyes. Then you realise this is actually for our sake and no one else's - it's not the President who has arrived."
It never went to their heads. Apart from the two bottles of French dry champagne, Johnny Walker Black Label Scotch and dry ice included on the request list to the production company on their Australian tour, Abba's requirements were disappointingly mainstream: tea, coffee, milk, fresh fruit. The truth was, Bjorn explains, that they'd have preferred not to leave Sweden at all.
Benny complained that on tour there was no time for songwriting, while the group had become so popular in countries like Australia and Japan that Agnetha and Frida couldn't leave the hotel except in disguise, spoiling any sightseeing and eating out.
"We had a lot of fun on stage but that was just a few hours compared to everything else," writes Frida in the book. "Plus I missed the children all the time and that didn't help."
Bjorn agrees that being a parent was at odds with touring as a band. "Agnetha and I had a baby daughter, so it was always difficult to decide whether to go."
While Halling's book, with its focus on tours, gives the impression that they spent much of the decade abroad, in fact they toured for just five months, turning instead to videos as a means of reaching audiences. As a result, Abba avoided over-saturating their fans: there were no unsold tours and their 10th - and last - album, The Visitors, was the biggest seller of their career.
It was when working on set for the artwork for The Visitors in an old artist's studio in Stockholm in 1981 that Halling noted a coolness had overtaken the band. Their lyrics were deeper and darker, addressing topics such as failed relationships and the loss of innocence - although still with the customary catchy harmonies.
"They'd been so much in love," she says. "But now I got a sense that what happens to painters was happening to Abba: they'd worked in a certain manner for a number of years and suddenly they were empty. The lust had gone."
Bjorn describes the change in mood as a "creative drain" that left them with no choice but to try something else. "That's how we all felt. We decided to do other things for a while," he says.
By 1981 both couples had divorced and Bjorn and Benny had remarried. Bjorn, who has five grand-children, has denied that their British No 1, The Winner Takes It All, of the same year was about his divorce and insists that the end of their relationships had nothing to do with the pressures of being in such a successful band. "That was part of life, not Abba," he says. "It's what happens to so many people who get married. It was never going to work."
In the years following the split - although they never officially called it that - they took advantage of the opportunities Abba had opened up, Benny and Bjorn working on the musical Chess with Tim Rice in 1983 and Agnetha and Frida relaunching solo careers.
"We always had the thought in the back of our minds that we'd get back together, but in the end we didn't," Bjorn says.
It took longer for the girls to move on in their personal lives. Frida, who now lives in Zermatt with her boyfriend Henry Smith, the fifth Viscount Hambleden, describes the break-up of the band as a wound that never healed. "Abba was our everything: our mutual relationships, our partner relationships, children, families, houses, moves and break-ups," she says.
In 1992 she married Prince Heinrich Ruzzo Reuss, Count of Plauen, only to be widowed in 1999. Her daughter Lise-Lotte from her first marriage, to Ragnar Fredriksson, had died in a car accident in New York the previous year; she has a son, Hans, but did not have children with Benny.
Meanwhile, Agnetha says there's still not a day when she doesn't think or dream about Abba. After withdrawing from public life in the 90s and setting up home on a farm on the island of Ekero in the Stockholm archipelago, she was stalked for many years by Dutchman Gert van der Graaf, with whom she'd had a brief romance. But she has come out of hiding and last year had a new album, A, and appeared in a BBC documentary, Agnetha: Abba and After ...
Enthusiasm for Abba waned during the 80s, but the group was back in the charts with Abba Gold, a collection of their greatest hits, in 1992 which went straight to No 1 in Britain.
In 1994 the film Muriel's Wedding, whose main character is obsessed with Abba, tapped into this renewed enthusiasm for the group but it was the musical, Mamma Mia!, that fuelled a new era of global Abbamania when it was first performed in the West End in 1999. It's the 10th longest-running Broadway show and the highest-grossing musical of all time and inspired the 2008 movie starring Meryl Streep, Pierce Brosnan and Amanda Seyfried.
At the Swedish premier on July 4, 2008, the Abba foursome came together publicly for the first time since the 80s and the movie soundtrack went to No 1 on the US Billboard charts, Abba's first US chart-topping album.
Halling, who runs the Abba Museum in Stockholm, which opened in 2008, was nervous about showing her book to the band. "If they hadn't approved, I wouldn't have published it," she says. "This is a small country and we are friends."
Along with photographs and anecdotes from Halling and the band, the book contains envelopes stuffed with memorabilia: Abba fans will be all over it while others will enjoy flicking through it. For Bjorn and the other band members it released memories that had been long-forgotten. "It's a brilliant idea because she remembers things that I don't," he says.
Still, it didn't prompt any nostalgia for the 70s. "It's a strange thing always seeing myself as a young man," he says. "The more it happens, the more abstract he becomes to me."
That's not to say he's not proud of what Abba has achieved. He insists that Abba is at the root of the world's present Scandinavian obsession. "Everyone tells me that's the case," he says. "And, yes, I think it's quite possible. Until we had our success it was still unthinkable that anything from Sweden would catch on. Now the music industry in Sweden is now the most successful per capita with America and the UK. We opened the door."
Surely, then, it's time for an Abba comeback tour? Agnetha has raised the possibility a couple of times. They'd make a fortune, just like the Rolling Stones did at their comeback gig. "Yeah, and whoever is richest when he dies, wins," Bjorn says sarcastically. "No, it's not going to happen."
ABBA: the details
• Abba are the 4th biggest-selling artists of all time, after Elvis, The Beatles and Michael Jackson.
• Dancing Queen and Fernando were Abba's biggest singles, and Arrival their hit studio album.
• 380 million albums sold worldwide.
• 14 Top 40 British singles.
• Abba Gold sold 28 million copies; the biggest-selling Abba album.
• Mamma Mia! is the highest grossing musical of all time.
• The Winner Takes It All is about the divorce of members Bjorn and Agnetha. Agnetha says it is her favourite Abba song.
Abba: The Treasures, by Carl Magnus Palm and Ingmarie Halling (Carlton Books) is available through Amazon.
