No one can be quite sure when Abba ascended from a critically derided band in Bacofoil costumes to highly respected pop sanctity. Despite the fact that the Swedish quartet has never reformed, Abba's resurrection just seems to gather momentum. That is undoubtedly why the Old Fruitmarket was packed with a crowd eagerly anticipating the UK debut of an obscure 15-piece Swedish folk "okester". The band includes spectacular musicians, some from the Stockholm Philharmonic. But the main attraction is leader and main composer, Benny Andersson.

Andersson has replaced the Abba classics with an entirely acoustic big band, performing in a traditional Swedish folk style. He plays lead accordion. It's like discovering that Ziggy Stardust-era David Bowie had been moonlighting as a Morris dancer.

Andersson looks delighted to be playing in front of a clapping, stomping audience again. Although there is not a Dancing Queen to be heard, Andersson's new/old music parallels his former band: it is a disconcerting blend of cheerful cheesiness and soaring, glorious melodies.

The opening tune, Hardangervidda, has an Abba-esque floating flute intro breaking into a swinging fiddle and accordion waltz. Anita's Polska becomes a rousing whirligig of sound akin to Swedish Zydeco. The joy of the band in full flight washes over the audience, and the effect is genuinely thrilling.

Helen Sjoholm, star of Bjorn and Benny's latest musical, Kristina, layers rather stilted vocals over jaunty folk rhythms, but it is a mainly instrumental set that reveals an insight into the origins of the Abba sound. It's all there - the marching polka tempos, the high, melancholic melodies and florid piano breaks. There are dips, especially in an ungainly number that sounds alarmingly close to Acker Bilk, but Andersson's loving return to his musical beginnings is a strangely exhilarating experience.