It’s hard to believe that there was a time in Britain when to admit to a passion for the works of Abba was — in certain circles — a form of social suicide. They arrived into our lives in 1974, at a time when ageing baby boomers were listening to Pink Floyd and their brattish offspring were preparing to pogo to punk. Abba’s catchy tunes and kitschy outfits symbolised all that was awful about middle-of-the-road pop. Yet 30 years later, we ’re all Abba fans now.

The Abba musical Mamma Mia! — written by the two men of the group and featuring all the band’s biggest hits — is celebrating its fifth birthday, and has been seen by 10m people worldwide. The newest convert is the Prince of Wales, who will see the show this week. Meanwhile, the group’s greatest hits album, Abba Gold, is busy making its own records, celebrating its 469th week in the UK charts. (Overall they’ve sold about 400m CDs and records since their first hit.) And next month thousands of fans from all over Europe will be attending the Abbafest in Brighton to celebrate the band’s Eurovision Song Contest victory of 1974.

But why is all this happening? How has it happened? Everyone seems to have forgotten the one fact that doesn’t show up in all the mind-boggling Abba statistics: the Eurovision Song Contest is supposed to be a one-way ticket to obscurity. But Abba won and then went on to win the hearts of the world.

In doing so Abba has defied what was once the central commandment of pop music: thou shalt be here today and gone tomorrow. Ye shall shine in sequence, sell a million records, enjoy your 15 minutes of fame and be forgotten. But no, not Abba. They are the band that time refuses to forget.

Abba’s longevity is one of the great mysteries of pop music, so I turned to Abba’s Bjorn Ulvaeus for some help. He was the one who played guitar and was slimmer than Benny. Okay, let’s try that again. He is the clean-shaven one (usually) and was married to the sexy-blonde one, Agnetha.

These days, aged 59, he is not clean-shaven. He has a furry, subtly grey-flecked beard and small, smiling teddy-bear eyes and a mop of luxuriously deep-brown hair. He lives with his second wife, Lena, in Sweden with their two kids, Emma, 22, and Anna, 18.

He is willing and able to put the British Abba obsession into a precisely scientific international context. “Abba are certainly more popular in Britain than southern Europe,” he says, “but compared with Sweden and Australia, they’re really about equal.” The Americans, he says, lag far behind: in Britain the band had nine number one hits; in the United States they had just the one, Dancing Queen.

But is there any country on the planet that has been able to resist the tuneful charms of Abba — North Korea, maybe, or Outer Mongolia? Bjorn chuckles: “No.”

“What about Africa? Deep in the Heart of Darkness? Not much Abba mania going on there?” I suggest. He is keen to disabuse me of such naivety. “On no,” he says, “A friend was recently out there in the bush on safari. He was enjoying the sun, the great silence of the plains when suddenly he heard the sounds of Dancing Queen.”

Perhaps one reason for the continued success of Abba is that in a Britain that’s so culturally fragmented, they’re the one thing that we can all agree on. Abba are chicken tikka masala for the nation’s ears.

One of the most fascinating things about the Abba consensus is that people — critics and snooty rock fans — who hated them back in the 1970s now claim to have “always loved Abba”.

Bjorn mulls the idea over. “It’s strange,” he says. “For those people we used to be the Antichrist. I remember when we split up in 1981 and Abba was still very uncool.”

Not only were the band uncool but on the brink of being forgotten. It was a fate that Bjorn and the rest had prepared themselves for.