That fateful April night when 500 million people in 32 countries watched a virtually unknown Swedish pop group beat 16 other entries to win Eurovision was so long ago, there was a guest appearance by The Wombles. But the winner takes it all. A quarter of a century later, almost everyone in the West can still sing along to "Waterloo", the winning song from Agnetha, Anni-Frid, Benny and Bjorn.
When gay clubs began replaying Abba's hits at the start of the Nineties - in suitably quasi-ironic fashion - they incited a best-selling cover version from the likes of Blancmange and Erasure. That led to Abba spear- heading the entire Seventies revival. The world may have stopped short of reclaiming their ghastly costumes, thanks to the first law of fashion - if you can remember wearing it the first time, don't even think about it the second - but compilation CDs like Abba Gold are selling by the truckload.
Small wonder that weeks before opening, advance sales for Mamma Mia, the musical based on Abba songs, were near the pounds 2m mark and climbing. The show, the brainchild of producer Judy Cramer, who worked with Benny and Bjorn on their hit musical Chess, has been in gestation for at least 18 months.
Previous attempts to dramatise their back catalogue (including the sublimely titled Abbracadabra) foundered, but after two weeks of previews, Mamma Mia opens tomorrow. One reason this version has legs is the calibre of its personnel.
For starters, the producers secured the designer Mark Thompson, the man behind the look, style and feel of the enormously successful revamp of Joseph and His Amazing Technicolor Dreamcoat and the visually entrancing Doctor Dolittle. Only after he came on board did they select the director Phyllida Lloyd.
Although this is her first musical, Lloyd is no stranger to the peculiar perils and pleasures of the form, having directed a punchy revival of The Threepenny Opera at London's Donmar. As well as endless plays, she's also directed riotously well-received operas. Indeed, she has only just finished Verdi's Macbeth. "When I arrived at the read-through immediately after working at the Paris Opera, in terms of concentration, intimacy, commitment and privacy, it was like entering a Peter Brook laboratory." Although she sees more similarities than differences between the forms, the biggest single difference is the musical's dependence on electronics and sound. "The massive presence of amplification means you have to try to match that power with the actors." Amid the brouhaha surrounding the news that the National Theatre is now using microphones, few have addressed the fact that, whereas musical performers once filled theatres with their natural voices, these days every musical is miked.
"Watching something like Saturday Night Fever, quite often you can't actually tell who's speaking because the sound comes from loudspeakers. So we're trying to go for perhaps rather old-fashioned values. This is not a hi-tech production. We've deliberately gone for a rather hands-on, we-do-it-all-ourselves sort of feel." So, no hydraulics then? Mark Thompson smiles, sheepishly. "Well... there is one element of hydraulics," he concedes. But Lloyd insists that his simple, versatile set has allowed for changes and developments during rehearsals.
Not only does this buck the trend towards spectacle, it also allows for an unusual degree of creative freedom. The grander the set, the earlier it has to be built, and that's usually ages before rehearsals begin. That militates against really major rewrites, a redesign being, at best, almost prohibitively costly.
As Thompson observes, the big difference between the forms is that opera is a given and can't be changed. "But with musicals, even when the songs are a given, as they are here, it can be tampered with." With Catherine Johnson's script now in its seventh draft - she's the third writer to have worked on the idea - there has clearly been more than tampering and, after four previews, the creative team are about to disappear to address one last big structural problem. "We're going into hiding for two days without the cast," announces Lloyd, nervously.
The previews have been extraordinarily instructive. "We were all really longing for an audience because it's such a peculiar piece in that they own half of it." Normally, you pray that audiences will go out humming the songs. Here, they go in humming them.
That brings its own difficulties. Stephen Sondheim has always contested that audiences don't hear harmonies, just melodies. Whatever the truth of that generalisation, it is manifestly not the case here. Audiences know these songs inside out. Only a tiny percentage of them could discuss the delicious harmonic suspensions of, say, Thank You For the Music, but everyone senses that it's not just the melodies that makes these songs work.
Nevertheless, to Lloyd's amazement, composers Benny and Bjorn have readily taken new ideas on board. "Yesterday, we were very worried about a particular strain of orchestration that we thought was preventing the audience from experiencing a particular moment in a scene. So they just said: `We'll rewrite it.'" The backing vocals which audiences know so well have created a particular conundrum. "Take them out because they're inappropriate to the scene and we've found the audience sing them anyway. So we put them back in. We keep trying different things in order to release the scenes in the right way." Finding a dramatic imperative for the songs has been the aim. "I'm not saying that every song is essential to the drama, but some of them move the story forward and express the inner life of the characters, while others come out of left field.
"The story is partly dealing with the Seventies... we needed that excuse to allow a certain cluster of songs." In stark contrast to the quick-fix, flimsy structures propping up most greatest hits-style musicals, Johnson has written a real plot about a girl on the eve of her wedding searching for her real father. Instead of carbon-copy retreads, several songs are sung by another gender or another generation. "The Name of the Game" is no longer about a relationship. It's now used by somebody asking someone to declare if they are their father. "Does Your Mother Know?" is now sung by an older woman to a very young boy trying to make a pass at her.
"We're trying to make you hear them in a new way," says Lloyd, rebuking those who wish to write it off as another cheap compilation. Let's face it, Abba can't be in it just for the money. In 1982 they were Sweden's highest foreign currency earners - more than Volvo. Disc sales are unimaginable and they get both performance and writers royalties.
A percentage of potential box-office millions isn't exactly a tough pill to swallow for Lloyd and Thompson, but they are remarkably free of cynicism. "We're trying not to take ourselves too seriously," says Lloyd, "but the ballads in particular are like little theatrical tales. We want to create an extraordinarily festive, witty, ironic, surprising bed for these wonderful songs, and to make a story that releases them in a sometimes surprising way.
"We hope to create pure pleasure. We're not splitting the atom."
Prince Edward Theatre, London. Box office: 0171-447 5400