Rona Munro: If Tim Roth hadn't bailed out...

Maybe you haven't heard of Rona Munro. She's a jack of all trades and a master of – well, some. An award-winning playwright, her latest stage offering kicks off at the Royal Court this week. She's a screenwriter, too, whose successes range from Ken Loach's Ladybird, Ladybird to the last ever episode of Doctor Who. But she started out, back in her native Scotland, as one half of a feminist comic-theatre double act, The MsFits. It's in this latter role that Munro, now 43, admits she may not have excelled. As a performer, she says, not exactly benefiting from hindsight, "I was shit".

It's not an epithet that could be applied to her playwriting career, the major achievement of which is her latest, Iron. The play describes the relationship between a jailbird mum and the daughter she hasn't seen for 15 years and it was the darling of last year's Edinburgh Fringe. It's an impressive work, which plays off the audience's expectations of "prison drama" to tell a powerful story of fractured families and criminal justice.

"I was writing more about family than about prisons," says Munro. She'd spent time in prisons when researching a screenplay commissioned by Tim Roth. When the project fell through, she was left with a clutch of vivid impressions of life under lock and key, which she then fashioned into a play of her own. "There were three things I wanted to explore," she says. "The attitude of someone going into prison, the attitude of someone in prison, and the whole issue around families. The thing is, you have this unbelievable boredom in prison. So when a bit of outside life comes in, it's extremely intense." It was the perfect crucible for a dramatic exploration of family, says Munro. "It's that thing of a mother living vicariously through a child, and the child needing to identify with the father as well as the mother," she says. "Iron describes an extreme form of those dilemmas."

Like Munro's debut Bold Girls, which was about Belfast wives during the Troubles, or like 1999's Snake, whose characters grappled with life on a council estate, Iron is a private story against a political backdrop. Its heroes are hard done by life. "I always think," says Munro, "I'm going to write something light and funny, but it doesn't happen." Her upcoming BBC2 film, Rehab, directed by Antonia Bird, describes life among the lost souls in a drug rehab clinic.

Characteristically, though, Iron resists polemic as resolutely as it does sentimentality (Ladybird, Ladybird remains, for example, Ken Loach's least ideological film). Munro explains this tendency in terms of her own modesty. "It's all very well for me to offer my opinion here to you," she says, "but it's another thing to say, 'and now we'll go into a theatre and pay £12.50 to hear my opinions'. Get out of it! You've got to tell stories that are more universal than that."

Iron's political dimension is integrated stealthily into the human drama. "I don't think the play really raises this except as a background," says Munro, "but it does seem to me that we don't know as a community what we want our penal system to do. Some people think it's for punishment, some people think it's for rehabilitation. And the actual system falls between those two stools." Watching Iron, we start judging the murdering mother, Fay, for ourselves. Does she deserve her punishment? "Is her crime one of those things anyone could do, but some of us don't?" asks the writer. "Or is there a type of person who does this? With the addition of drink or drugs or a particular set of circumstances, could we all just act like that? Because I know I've been angry, and destructive, even towards people I love. It's always someone you love, isn't it?" She pauses. "Mind you, when I say that, there's often a glassy look in the eye of the person I'm speaking to. They either identify or they go, 'no, you're a crazed psychotic whore'."

This searching imaginative sympathy is again characteristic of Munro's work, and it's a quality that can be traced back to her performing roots. "I was communicating very directly with actors from the word go," she says. "And if a writer has an understanding of what performers have to do, then they write lines that people can say." Munro was still a performer when her TV writing career began, first with work for local telly then, in 1989, on Doctor Who. It was her first script for the BBC; she didn't know that, after 26 years, it would be the sci-fi series' last. "What a lousy ending for the last one," she says now, ruefully. "The BBC were so determined to axe it. But you think, 'Oh God, if only we'd made it really special, maybe it would have been saved.'" But her own career soldiered on. The MsFits, which she co-founded with the actress Fiona Knowles, went from strength to strength. Since Munro's retirement from performance, the pair have made a stream of one-woman plays together – Munro scripts, Knowles performs. It's a far cry from the 1980s, says Munro. "When we started, there were women's cabaret events, and Latin American solidarity events and Help the Miners gigs – there was so much theatre of an overtly political, get-up-and-do-some-comedy nature. And it's all gone."

Does she lament its passing? "Yes I do," she says. "But things have to move on, and you kind of hope that in 20 years time, if we haven't all blown ourselves up by then, things may have moved on again." She doesn't regret, however, having progressed beyond the political certainties of her own youth. "My writing's got more mature in the breadth of experience it shows," she says. "It's very reassuring to have political certainty. But when you get on a bit, that certainty evades you. And yet, in a way, that's a stronger thing: to say important things but not make them dogmatic statements."

Which brings us back to Iron: thoughtful, substantial but never partial. "The only certainty I do feel that I have at the bottom of my work," concludes Munro, "is some kind of belief in the triumph of the human spirit." Then she stops, and cringes, because "this sounds so pretentious. But bleak as the subjects often are, or bleak as the situations often are," she says, "people are being pretty amazing within them."

'Iron': Royal Court Downstairs, London SW1 (020 7565 5000), now previewing, opens tomorrow to 1 March