'What's gallus?" Catherine Johnson asks, unprompted.

The writer behind ABBA-based hit musical Mamma Mia! is contemplating how one of her characters for her earlier play, Shang-A-Lang, has been described to her by the team behind Rapture Theatre's touring revival, and isn't sure how it translates into her own West Country patois.

When it's explained to Johnson that somebody who is gallus is someone with attitude, swagger and cheek in abundance, it seems to hit the spot.

"That's Lauren," she says of one of three middle-aged women in the play who go on a bender at a 1970s revival weekend at Butlin's holiday camp, where a Bay City Rollers tribute act are headlining. Over the course of the weekend, Lauren and her pals, Jackie and Pauline, have assorted epiphanies as they encounter a couple of equally ageing rockers.

"I'd been thinking about writing a play set in a holiday camp for some time," Johnson explains about the roots of Shang-a-Lang, which first appeared at London fringe theatre The Bush in 1998.

"I'd been to Butlin's in Ayr for a cheap holiday, and then my sister went on one of these 1970s weekends, came back and said 'that's what you've got to write about'. I mean, tribute bands: what's not to love?"

She went on a reconnaissance expedition to Butlin's in both Bognor and Minehead, with then Bush artistic director Mike Bradwell in tow.

"Mike bottled out after a day and went home," Johnson says of the man who championed her writing. "But we did see a version of the Bay City Rollers, Les McKeown's version, I think. We also saw The Sweet, not long before [singer] Brian Connolly died, and we saw Desmond Dekker, who was amazing. It isn't just booze, sex and vomit. You can have a good time as well."

Johnson's career as a playwright began when, inspired by the likes of Jim Cartwright's play Road, she entered a playwriting competition run by Bristol Old Vic and ITV station HTV West. She won this with Rag Doll, a play about incest and child abuse which was staged at Bristol Old Vic in 1988, and filmed for TV.

"Success for me was writing something from start to finish," Johnson says, "but I won, and there's nothing better than being told that you can do something."

She sent her next play on spec to The Bush. Bradwell's staging of it began their long working relationship, with Johnson penning three more plays for Bristol Old Vic in-between writing for TV on the likes of Casualty, Byker Grove and Band Of Gold. However, it was Shang-a-Lang and, especially, Mamma Mia! that put her into the mainstream as much as the book that Shang-a-Lang was partly inspired by - Helen Fielding's novel Bridget Jones's Diary.

"It seemed to be saying that if they haven't got a man as an appendage, all women are useless," Johnson explains. "Me and my mates at school were all rampant feminists, or so we thought, even though we all secretly wanted boyfriends, but I was annoyed by the phenomenon of Bridget Jones."

Johnson hasn't read the new Bridget Jones novel but she is amused by one of the book's plot twists. "My God, she's killed off Darcy," she says dryly. "What a great thing to do. You've got to admire her chops for doing that."

Johnson's play wasn't the first Shang-A-Lang to make it to the stage. In 1987, Clyde Unity Theatre produced a play by Aileen Ritchie of the same name which was also about a group of Rollers fans. While it is unlikely Johnson had heard of Ritchie's play, the surface similarities between the two demonstrate just how much the Edinburgh-sired boy-band affected a generation's collective psyche.

This is also the case with jukebox musicals, a trend which Mamma Mia! pretty much kick-started. While some are snobbish about the commercially-minded melding of contemporary narrative with already existing hit records, the term dates back to the 1940s with Judy Garland vehicle, Meet Me In St Louis, and one could cite films such as Rock Around The Clock and Beatles flick A Hard Day's Night as fitting into this category. As with the craze for rock'n'roll musicals such as Buddy and Return To The Forbidden Planet, the current wave of such musicals has its roots in fringe theatre.

Johnson's relationship with The Bush is testament to this, as is the success of Sunshine on Leith, Stephen Greenhorn's Proclaimers-soundtracked play which, like Mamma Mia!, has been turned into a film. Like Mamma Mia! and Shang-a-Lang, Sunshine on Leith has a common touch akin to a popular TV drama, with the naturalism broken up by the songs. "I get really annoyed when people talk about Dexter Fletcher's Sunshine On Leith," she says. "It was a stage play first, and it's Stephen Greenhorn's."

As for Shang-a-Lang, that remains very much Johnson's. "I was working on both Mamma Mia! and Shang-a-Lang at the same time," she remembers, "so they reflect each other. If I'd been doing a lot on Mamma Mia!, it was quite a relief to get back to Shang-a-Lang with Lauren, Jackie and Pauline and let them be as foul-mouthed as they liked.

"It's a play about friendship, and realising the people who are your friends aren't necessarily the best people to be around after a certain time. They can hold you back. If I was to identify with any of the women in the play, it would probably be poor old Pauline, who's on her own."

Fifteen years on, Johnson retains a fondness for the play.

"It's like a gawky adolescent now," she says. "but it was so much fun. After Mamma Mia! became what it became, that kind of became my identity.

"That was fine, as there was no point in denying that I wrote Mamma Mia!, because it brought me so many benefits, like getting to know the people who wrote all those wonderful songs, but Shang-a-Lang is still me as well. Now that it is a gawky adolescent I may be mortified when I see it again, but looking back at it now, it brings out the gallus in me."

Shang-a-Lang, Brunton Theatre, Musselburgh, Thursday-Friday; Regal Theatre, Bathgate, November 11; Albert Halls, Stirling, November 15; King's Theatre, Glasgow, November 19-23, www.rapturetheatre.co.uk