Waterloo was the song that thrust them onto the international stage back in 1974 - the incomparable Swedish quartet, ABBA. They haven't played together for over three decades. But this week Anni-Frid, Benny, Björn and Agnetha proved they can still pull in the crowds, when the ABBA Museum opened in Stockholm. To find out more about what's on show, Helen Seeney spoke to curator Ingmarie Halling.