Catherine Johnson: Who wants to be a millionaire (and write plays)?

I first met playwright Catherine Johnson four years ago; she had written the book to the Abba musical Mamma Mia! and the show was about to open in the West End. It went mega across four continents and the struggling author, like a few others associated with the show, became a millionaire almost overnight. So it is with some trepidation that I meet Johnson again, as Little Baby Nothing, her first new play since 1998, is about to open. Would I find that her head had been turned by money and success?

I should have known better, of course. Johnson has chosen to premiere the new work, not at the National, or a West End theatre where she would have been welcomed with open arms, but at her much loved Alma Mater, the tiny above-a-pub Bush Theatre in west London, where her first three plays – including the wonderfully raucous hit Shang-A-Lang, about a hen weekend at a Butlin's where a Bay City Rollers tribute is being staged – were performed. How could I have thought that a few quid in the bank might have transformed a shy but witty interviewee into a limelight-grabbing harridan?

Johnson hoots with laughter at the suggestion and replies, in typically ironic tones: "Yes, you've noticed the designer clothes, then. Well spotted." In fact, she is casually dressed for the play's rehearsal in a church hall, a venue which causes her much amusement. "The vicar has popped his head round the door a couple of times, but not when we've been doing the Satanism stuff, thank God. He may be trendy but I think it would be a bit much even for him."

So what is Little Baby Nothing about, I ask. "Oh, this is the part when I always sound like a pretentious prat," Johnson says, although her West Country vowels make it difficult for anything she says to sound inflated. Put it in your own words, it'll sound so much better."

For the record, Little Baby Nothing is about three Bristol teenagers who are into sex, Satanism and cider, and the single mother of one of them. "And yes, she is me, or at least aspects of me," says Johnson, who grew up in Gloucestershire and was into punk in her formative years. Although aspects of Little Baby Nothing are quite dark, it is replete with trademark Johnson ribald humour. "We've been laughing a lot in rehearsal. I think the actors are laughing because the lines are funny rather than because they think they're crap."

Johnson, 44, has two teenage children and freely admits to plundering their lives for her new play. "It's the first time I have written about something that is so directly about part of my life. All through my working life I have had two children but I wasn't ready to write about it. It was writing about the mother-daughter relationship in Mamma Mia! that made me realise I wanted to explore the subject more deeply."

She has drawn on her own childhood – "Cider-drinking does seem to be a rite of passage that never changes" – as well as her children's. "I asked my daughter [who is now 15] to come to the Las Vegas opening with me. I thought she would jump at the chance, but strangely she didn't. It was only when I returned that I realised it was because she was throwing a huge party about which I had no inkling. We shall say no more."

Money, as for most playwrights, was an issue for Johnson before Mamma Mia! She didn't start writing until she was 30 – "I just felt compelled to" – and was a single parent for several years. Since the musical's success, she has moved to a bigger house, although in the same area of Bristol, and the children still attend local schools. Life is certainly more secure.

What she never has to do again, though, is take commissions for non-stage projects to keep a roof above her family's head. "I'm now in a position to say, 'Fuck that' and turn down work. I've got a regular income now." Enough to retire on? "Possibly, yes, if I did without the yachts," she deadpans. So is she rich, or just comfortable? "How do you define it? To me it's a fuck of a load of money. If I died my kids would be OK and it's only after this happened that I realised just how much anxiety I used to have over money."

So does she feel she has changed? "I honestly don't think so, although I'm not the one to judge, of course. I've still got the same friends I had before this all happened." What about the glitz? Does she enjoy attending the many premieres that Mamma Mia! has had around the world, from New York to Sydney, Los Angeles to Tokyo? "Well, no one famous has ever rung me, but I couldn't miss a party, obviously."

The one change I have noticed is that she's more relaxed during this interview. "I suppose so, yes," says Johnson. "I've done so many now that I don't get nervous. But I'll still be crapping myself on the first night that people will hate the play."

'Little Baby Nothing': Bush Theatre, London W12 (020 7610 4224), Wednesday to 21 June