‘His situations and his characters have a theatricality about them automatically, which seems very tempting initially; the trouble is that from the practical point of view, he throws up all sorts of challenges.” Thus playwright David Wood about adapting the works of Roald Dahl for the stage.

And Wood should know: George’s Marvellous Medicine, which comes to the Playhouse next week, is his sixth Dahl adaptation for audiences aged four and upwards. Published nearly 30 years ago, it’s the one about a boy who invents a medicine to cure his grandmother’s appalling temper. Needless to say, there are side effects.

“When Grandmother actually takes the magic medicine,” Wood told me, “she has to grow, grow and grow until she crashes out through the ceiling — with her feet on the floor and her head and upper body poking out of the roof — where she’s really rather happy for quite a long time!” I said I wouldn’t ask how that’s achieved.

“All I can say is that you do see it; it’s a great moment and you use a lot of purely technical theatrical magic to achieve it.”

This new adaptation was a commission from the Birmingham Stage Company, and for once, Wood isn’t directing as well: that task is down to Phil Clark, who directed the Wood version of Danny The Champion of the World (seen at the the Playhouse in 2008).

The plot of George’s Marvellous Medicine is fast-moving and fact-filled. But that is a positive advantage for the children in the audience, Wood says. “George uses them and they become his friends; and particularly in the second half they have to remember all sorts of ingredients he has used in his medicine. It’s extraordinary how much children retain, far more than adults: they remember things from an hour or two earlier that the grown-up has no idea about.”

I just want to see how granny grows.