For all that, their music remains a curious anomaly – a universally successful sound based not in the riotous, sexualised lingua franca of rock or r’n’b but in more homely, restrained and European values. If rock’n’roll is about escape and outsiderdom then Abba’s pop songs excavate the dreams and disappointments of those who never wanted to rebel. Summed up in the tumultuous ecstasy of Dancing Queen or the cascading fatalism of The Winner Takes It All, suburbia’s hopes and fears are no less epic and no less valid. And there are an awful lot more people living there.

Swede dreams

“The key to Abba is their understated Swedishness,” explains Carl Magnus Palm, author of the definitive biography of the band Abba: Bright Lights, Dark Shadows. “Specifically it’s the importance to their music of Swedish folk songs and of a sound called Schlager, which means ‘hit’ in German.” Bereft of rhythm and blues or soul influences and cloyingly kitsch and sentimental to modern ears, the central European sound of Schlager was vastly popular – the soundtrack to a million camping holidays and cabaret evenings. “It’s not cool music,” says Palm, “but Abba grew up loving Schlager and they added other things that they loved like The Beach Boys and The Beatles to that simple blueprint. Millions of people related to it.

“And there’s also a melancholy Nordic streak even in Abba’s happiest songs. Even in Mamma Mia there’s some extra layer of sadness that you wouldn’t hear in a contemporary record by, say, The Brotherhood of Man. There’s a depth to Abba that hints at deep sadness.”

That sadness was not imagined or confected. All four members of Abba experienced private sorrows as they grew up in postwar Sweden, from family poverty to parental coldness and debilitating shyness. Both Benny Andersson and Anni-Frid ‘Frida’ Lyngstad became unmarried parents before they reached 20; each had to make the decision to leave their children behind to pursue music.

As Abba’s success grew, Benny (beard) married Frida (dark hair) and beardless Björn Ulvaeus married blonde Agnetha Fältskog, only for both couples to divorce towards the band’s end. These marital breakdowns imbued Abba’s final albums Super Trouper and The Visitors with a very adult pathos and resignation, qualities which dismissed critical charges that the band made only vapid good-time pop. But Anni-Frid’s unhappiness was of a different order. “Frida’s background is so painful that you wouldn’t believe it if it was a work of fiction,” says Palm.