Next to Sing-a-long-a Abba, London's other Abba-fest Mamma Mia! has the psychological complexity of Twelfth Night. But the newcomer also has a tacky, happy charm all of its own that may not be the West End as I like it, but is certainly as many seem to.

Though I've never been on an 18-30 holiday and am now a decade too old to even try, this is the kind of thing that I imagine its holiday reps encourage their customers to get up to on the Costa Blanca. To the accompaniment of a live and happily ludicrous tribute band, you're encouraged to let your hair down (or wear a period wig) and surrender to the party atmosphere: "Friday night and the lights are low, /looking out for a place to go/ Where they play the right music getting in the swing"?

If that's you, you'll find that place right now at the Whitehall Theatre, and you can get interactive with some of pop's most insistently memorable melodies. It's also interesting to notice, when seeing Abba's lyrics projected onto a video screen, how unbelievably silly many of them are! But then this isn't meant to be an occasion for literary analysis. Instead, you're not only actively encouraged to sing, but also to regularly dance - there's even an onstage dancing competition to crown that night's Dancing Queen (won on the first night by a burly bearded guy called Tim). And thanks to a free goodie bag given out to every patron, there's some more interactive 'magic moments' along the way.

The concept for these sing-a-longs began when subtitles were added to the songs for screenings of the film version of The Sound of Music (still playing at the Prince Charles Cinema, off Leicester Square) and later extended to screenings of Joseph and the Amazing Technicolor Dreamcoat, but I'm sure the idea was inspired by the spontaneous audience participation that made The Rocky Horror Picture Show a cult.

If this seems to be altogether more manufactured and contrived than that ever was, a good time nevertheless seems to be had by (nearly) all. Next up, the producers are threatening sing-a-long-a Fiddler on the Roof.

- Mark Shenton