The Angel Gabriel wants to be Mary. Herod wants to be David Beckham. Baby Jesus’ head just fell off, and the innkeeper’s a budding psychopath. Ten years ago, Tim Firth’s Flint Street Nativity, a look at the chaos backstage at a primary school nativity play, was conceived as a one-off, 70-minute television play for the Christmas schedules, a vehicle for the comedy talents of Frank Skinner, Neil Morrissey and Dervla Kirwan, doubling up roles as the warring six-year-olds in teatowels, and their parents. Those stars have all moved on over the last decade, as has Firth himself, who has been busily occupied writing the screenplays for films like Calendar Girls, Confessions of a Shopaholic, and Kinky Boots.

However, due in part to festive scheduling re-runs, the kids of Flint Street stuck in people’s minds. They got a second lease of life when Firth was asked to adapt the production for the Liverpool Playhouse in 2007, and this year they’re developing Edinburgh accents and moaning about the tramworks, as Firth has recruited the likes of Colin McCredie (Taggart) and Julie Wilson Nimmo (Balamory) and relocated the play to the Festival Theatre and the (fictional) Corstorphine Road Primary School.

‘I wanted Edinburgh to feel as great a sense of ownership over the play as Liverpool did with the original stage version,’ says Firth, explaining why he felt the need to be personally involved in the transfer. ‘The references and tone were all very Liverpudlian, and it was important for me that the play could break that link, that people in any town, any time would be able to relate. That was very important to me. It’s not just a question of changing local references and place names: so much of the comedy is to do with the music of the way people speak and the peccadilloes of language that we use in everyday life.

‘I’m from England – I can’t write that. I needed to be in the rehearsal room with actors who are Scottish, listening to the cadences and asking them: how would you say this if you were six years old and living just off the Corstorphine Road?’

Audiences familiar with the television version will also remember how compactly it packed ten very familiar characters – from the power-mad school bully in the angel wings to the lisping Wise Man with stage fright – and hints of their affecting home lives into just over an hour. In order to draw things out for the stage, Firth has turned the nativity into a musical.

‘We never really got to use the carols in the original, and I think that’s a shame. If you’ve got a load of kids singing words they don’t understand like “lo he abhors not the virgin’s womb”, then their minds are going to wander, so I realised I could essentially make the carols into soliloquies, where you can unlock what’s going on in their home lives, so you get to spend time with the children. And once they’ve told us all the truth about what’s going on in their lives, the parents come on and put that marvellous social sheen on everything, that usual load of adult flannel.’

If the production has endured, is it perhaps because the characters, both child and adult, are instantly relatable?

‘That membrane between the childhood and adult life is so flimsy anyway,’ he agrees. ‘There’s very little that goes on in the classroom that won’t happen 30 years later in the boardroom.’

Corstorphine Road Nativity, Edinburgh Festival Theatre, Thu 3–Sat 19 Dec.