A full-blown revival of the '70s pop music group ABBA ("Dancing Queen") is underway. Yesterday crowds spilled out on to a London West End street to catch a glimpse of stars as they arrived for the world premiere of an ABBA stage musical called "Mamma Mia!" The group's songwriters Benny Andersson and Bjorn Ulvaeus are behind the show, which takes their old songs and absorbs them into a new context. Tuesday was exactly 25 years to the day since ABBA won the Eurovision Song Contest, the event that propelled the Swedish music group to international stardom. Also marking the celebrations are two ABBA record releases: their "ABBA Gold" greatest hits and "ABBA: The Singles Collection 1971-1982," a limited edition boxed set of 29 CD singles. The latter is expected to become a collector's item. Only 20,000 will be released worldwide.