Björn Ulvaeus and Frida Lyngstad are sitting in a London hotel bar remembering the decadent 70s – that era of drugs and debauchery during which their band Abba hit astronomical heights of fame.
"And the strange thing is," says Björn, turning to his fellow Abba mate, "can you remember ever being approached by someone who came up to us and said," – his impression of a shady drug dealer at this point is so comically bad that you can only imagine the story he's telling is true – "'Hey look, I've got some really nice drugs here?'"
"Oh no!" shrieks Frida. "Never!"
"Never!" Björn bursts out laughing: "Never! Not even on tour! It's amazing isn't it?"
It's pretty unusual!
Frida: "Well we were at home a lot so they would have had to come to our houses and knock on our doors to offer us drugs!"
"Squeaky clean!" says Björn, still laughing. "But it's all true."
Meeting Abba is a lot like listening to Abba. They're instantly likable, lots of fun and entirely unconcerned with appearing cool. But, like their music, there's a lot more to explore when you scratch the surface. They don't tend to hang around, either: schedules are tight when you've sold almost 400m records, and even the minute-long breaks between various TV and radio interviews involve Björn autographing copies of the new book, with the swift, one-handed flourish of a man who's had to sign an awful lot of things.
And where to even begin with Abba's story? They were a band comprised of four couples – two in the married sense (Björn and Agnetha Fältskog; Frida and Benny Andersson), and two more if you include the unique vocal pairing of the women and the extraordinary songwriting partnership of the men. They had their Waterloo moment at Eurovision in 1974 before defying their destiny as one-hit wonders with a series of records that became hits across the globe – you imagine even those people camped out in obscure patches of rainforest still believing they were fighting the second world war owned a copy of Arrival.
As Frida says at one point: "The music scene changed with us – something like Abba didn't exist before; pop like that was not invented yet." Such was Abba's pop prowess that even the divorce of both couples couldn't derail them, at least not until after they'd written some of their best material, including The Winner Takes It All – which boldly, some say perversely, documented said divorce – and their final album, 1981's The Visitors, which tackled subjects as eclectic as cold war paranoia (the title track) and the pain of parenthood (Slipping Through My Fingers).
Global domination was never supposed to be in the script for a band who grew up absorbing influences completely out of sync with rock'n'roll trends: Swedish accordion music, Italian ballads, German schlager. For a while after Waterloo it looked like it might not be – their Phil Spector-influenced single Ring Ring was pretty much ignored in the UK.
"If you look at the singles we released straight after Waterloo, we were trying to be more like the Sweet, a semi-glam rock group," says Björn. "Which was stupid because we were always a pop group."
When Abba hit their stride, though – SOS, Mamma Mia, Fernando – they became unstoppable. It's well known that when Benny played Frida the backing track for Dancing Queen, to this day one of the most perfect pop songs ever written, she burst into tears: "And that was before me and Agnetha had even sung on it!" she smiles. "I knew it was absolutely the best song Abba had ever done."
Most remarkable about the song's magic – the piano trills (famously ripped off by Elvis Costello for Oliver's Army), the spiralling strings, the way it encapsulates a sense of uplifting joy – is that it sounds utterly effortless. So effortless, that critics at the time complained that the band were nothing more than a cold, clinical hit factory writing songs to order, with no heart. It's a criticism that Björn says used to make him mad, and possibly still does.
"Waterloo, Mamma Mia, Fernando, Dancing Queen, The Winner Takes It All … are they made to a formula?" he asks. "What is that formula?! It's totally the opposite. We never repeated ourselves. We worked so hard to find different styles every time."
Indeed, a tireless work ethic seems to be one of the secrets to Abba's success. Björn and Benny would take holidays just to write songs, and refused to leave a track unfinished: they would work and work on it until it was good enough, before turning their attention to the next one. They took inspiration from the Beatles by writing every song as a potential hit single – only when they had enough for an LP did that become the album. So intense were their studio sessions that engineer – and Abba's "fifth member" – Michael Tretow told Mojo in 1999 that he was often kept so busy he felt close to starving: "When there were red skies passing before my eyes and I'd be almost fainting they'd finally say, OK, let's break for something to eat!"
"Michael did eat," says Björn today. "He once ate two quarter-pounder cheese burgers in eight minutes. So he ate but he had to eat quickly!"
As the 1970s progressed, Abba seemed almost detached from the changing musical landscape around them. At times they would embrace trends – such as their disco album Voulez Vous, on which they finally introduced a groove to their sound – while at others times, such as when punk arrived, they would simply ignore them. Björn says he never felt threatened by punk because Abba were "so completely different", but in truth they had a lot in common with the movement. Both shared a healthy disdain for the excesses of progressive rock that had dominated the early 70s, both focused on brevity and both viewed the holy grail of pop to be the seven-inch single.
Björn smiles when I ask if he thought punk was a bit of a racket: "Well, I never quite understood it. There was a musical element missing. The rage, I could hear that. But young men have always been angry, that was no different from other young men."
Frida nods. "Punk never got into my heart. You hear the anger now in rap, for example, but it's different and I like that very much. Eminem is one of my favourites."
Really?
"Cleanin' Out My Closet is a great song!" agrees Björn.
If punk didn't topple Abba, then something closer to home looked bound to. In 1979 Björn and Agnetha announced their separation. Within two years, Frida and Benny were also divorced. Astonishingly, they carried on – much to the intrigue of their fans and the media. One live review from ZigZag in 1979 records Björn introducing Agnetha onstage as "my former wife" which seems unimaginably awkward. "Did I really say that?" he says, looking shocked.
"I think it was more 'And this is a girl I know very well,'" says Frida, which only proves that no way of skirting around the subject could ever make this situation appear normal.
Frida remembers her own way of dealing with the split from Benny: unable to leave the band, she simply reinvented her image. "I changed my whole style. I cut my hair very short, you know, very spiky and I became another woman in a way. So it manifested itself mostly like that."
Given that they'd made enough money for life by this point, didn't it make sense to do what every other band would do and quit?
Björn shakes his head: "We felt like we had something so valuable in the group that, even though it was difficult, we didn't want to break that up. And to prove it, we did some of our best stuff after that."
This "best stuff" happened not in spite of the divorces but because of them. Abba's early lyrics might not have been much much to write home about, as Bang-A-Boomerang can attest ("Like a bang, a boom-a-boomerang/Dum-de-dum-dum be-dum-be-dum-dum/Oh bang, a boom-a-boomerang/Love is a tune you hum-de-hum-hum"). But as Björn toured and broadened his reading in English he began to expand his lyrical palette, dealing with bolder and more personal subjects. The band became known for their ability to counterpoint joyous melodies with melancholic, even depressing, lyrics. If It Wasn't for the Nights summed up Björn's bleak state of mind during his divorce, a disco song with a lyric of utter despair in which the protagonist dreads the end of the working day, when they will be left alone to deal with their own thoughts: "There were times that last autumn I was with Agnetha that I had those nights myself," he admits. "My lyrics were often based around fiction, but that must have been where that one came from."
Crumbling relationships began to form the basis of many of their songs, from Knowing Me, Knowing You to When All is Said and Done, which was written specifically about Frida and Benny. So, how did Frida feel having to sing about the her own relationship?
"Well when you did it, you made sure you did it very professionally," she says. "Of course there was a lot of emotion behind it and it was not always easy to continue recording."
For the Winner Takes It All, Björn famously wrote about divorce as a competitive act featuring triumphant winners and fallen victims. The fact he then arranged for his former wife to sing it has sometimes been portrayed as an act of sadism, although he begs to differ: "No, not at all. I think she loved those words."
"She did," agrees Frida. "And remember that song was for so many people, not just Björn and Agnetha."
"And it was fiction, remember," says Björn. "There were no winners in our divorce."
The tribute bands (Björn Again), cover versions (Erasure) and hit musicals (Mamma Mia) may keep Abba in the public eye, but it is this emotional depth that has kept the band in people's hearts. Unfashionable at their peak, Abba have spent the time since they stopped recording slowly moving away from being the kind of pleasure people class as "guilty". These days musicians from Björk to Noel Gallagher are happy to praise them. Still, it seems unlikely that they will ever be held in the same regard as their heroes, the Beatles or the Beach Boys – not that this appears to bother them.
"I think being Swedes we have a very down-to-earth way of looking at ourselves and what we do," says Frida. "We've never had any, what do you call it … hubris?"
"Coming from Sweden, we were always regarded as outsiders, we were never part of that scene," says Björn.
"I recently read Graham Nash's [of Crosby, Stills & Nash] Wild Tales," adds Frida, "and to compare the lives we led and the music we wrote and the tours we did to that." She starts laughing. "It's just so totally apart from that scene, but it was a very interesting book. He really writes openly about drugs – all kind of drugs – and I suppose that was the environment there and then. But we didn't live in it."
Is gratifying to see their music gradually get reappraised by "cooler" artists, though?
Björn looks nonplussed: "I have to say I've always been much more impressed by the fact that millions of people all over the world buy your records. For me, there's no comparison."
Frida: "It does feel satisfying, I must say, that [modern bands] regard us like that, to hear we were the best pop group ever, for me it's wonderful to hear."
Do they keep up with modern pop? "I hear a really good pop song every now and then," says Björn: "ROAR by Katy Perry, I love that! Poker Face … oh! What a song! And Rolling in the Deep … oh!"
Does it make him jealous to hear a great song? Or at least competitive? "Oh, jealous, definitely! But I realise it's a young man or woman's work to write pop music," says Björn.
It is precisely this realisation that has led the band to decline numerous offers – some reportedly of up to $1bn – to reform. Björn believes that fans would ultimately be disappointed at the sight of four aged musicians on stage. But, given that they are a studio band first and foremost, I have often wondered why they don't record together. The prospect of an older, wiser Abba album is a tantalising one.
"It's difficult to talk about this because then all the news stories will be: 'Abba is going to record another song!'" says Frida. "But as long as we can sing and play, then why not? I would love to, but it's up to Björn and Benny."
Last year Agnetha said it was something she would also love to do. Could this be their last chance before – to paraphrase one of their own songs – time slips through their fingers? "Nothing's planned and it would have to be something very special," says Björn "But yes, why not?"
Later in the evening, Björn and Frida attend a party in the band's honour at the Tate Modern. Abba The International Party celebrates 40 years since they won Eurovision and is focused on the crazy outfits and that enduring appeal that means Dancing Queen will always be the first song on any sensible wedding DJ's playlist. Yet the event also reminded me of something Björn said when I asked whether he ever felt perverse writing his songs of heartbreak and despair to such joyous music.
"The music of Abba is not that happy," he said. "It might sound happy, in some strange way, but deep within it's not happy music. It has that Nordic melancholic feeling to it. What fools you is the girls' voices. You know, I do think that is one of the secrets about Abba. Even when we were really quite sad, we always sounded jubilant."
Waterloo Deluxe Edition, Gold 3CD Collector's Edition and Official Photobook are out now.
• Abba: from Eurovision to the split – in pictures
