In the next seats at the Birmingham Repertory Theatre, an impassioned discussion begins. “Is Oxford a real place?” a young lad asks his mother. She replies that Oxford does, indeed, exist. He remains unconvinced — after all, we are watching the stage version of Philip Pullman’s His Dark Materials trilogy, and everything else about the storyline is imaginary.

It’s difficult for me to remain silent, and neutral: should I be snootily indignant that anyone could believe that Oxford doesn’t really exist, or is it a great compliment to Philip Pullman that a youngster could assume that His Dark Materials starts in an imaginary city?

Philip Pullman roared with laughter when we met up the next day at his home just outside Oxford, and I told him this story.

“I’m glad to take it as a compliment! Of course, Oxford does feature in many works of literature, starting, I suppose, with Alice in Wonderland. I’m happy to be putting my little contribution into the myth of Oxford.”

Lyra, the 12-year-old heroine of His Dark Materials, lives half-wild in the dustier corners of an Oxford college. A sharp debate breaks out amongst the dons about her future. I wondered whether Philip was here having a bit of a dig at academia, having himself taught at the sharp end, in Oxford secondary schools?

“I didn’t set out to send academics up so much as just play with the natural propensity for the fantastical that overtakes anyone in Oxford. When The Amber Spyglass [the third and final book in the His Dark Materials trilogy] was published, the All Souls Mallard procession was taking place: every 100 years they take this wooden drake out of a cupboard, and march it round the quad, singing the Mallard Song, to commemorate something that happened around 1600. Apparently, one of the fellows told me, this last time they decided to have a duck as well, on grounds of sex equality. That’s something that could only happen in Oxford.”

At one point in the His Dark Materials story, an Oxford bus shelter becomes the starting point for a journey not to Barton or Kidlington, but to a whole new world. I wondered if Philip Pullman’s imagination simply fired up each morning, or whether he needed to walk around, looking afresh at everyday objects?

“It’s just a habit of thought, I suppose, a way of looking at things I’ve always had. Sunderland Avenue, between the Banbury Road and Woodstock Road roundabouts, has always fascinated me because the hornbeam trees there are so peculiar.

“It’s always seemed to me that something unlikely could well happen along there. And at the time, I was living just round the corner, so I could just go and check the trees out, and see what they looked like at different times of day. Everything you see, hear, overhear, read, you think, ‘what can I do with this?’.”

You suspect that courting controversy doesn’t come naturally to Philip Pullman. “It’s been comparatively late in my life when people have been prepared to listen to me on any subject at all,” he told me with a chuckle.

But he hasn’t been afraid to raise his head above the parapet, whether as a strong supporter of the campaign to save the Castle Mill Boatyard in Jericho, or when attracting criticism over the portrayal of organised religion in His Dark Materials.

Famously, he campaigned for the casting of Nicole Kidman to play the dangerous Mrs Coulter in The Golden Compass, the film of the first His Dark Materials book.

I asked Philip how much he’d been involved in the two stage productions — first at the National Theatre, and now a less elaborate Birmingham production.

“I haven’t got time to be involved at every stage. Also, there’s the matter of tact: you don’t go in and throw your weight around, saying, ‘you can’t do it like this, you’ve got to do it like that’. You plant suggestions, and hope they’ll be carried out. As for Nicole Kidman, it’s wonderful to sit around, have dinner, and suggest names for particular parts: of course, you come up with the most expensive and glamorous ones. But Nicole Kidman turned out to be absolutely right for the part, and it’s a great shame that it looks as if the films of the two later books won’t now be made, as a result of the credit crunch.

“At the National, Nicholas Wright, who wrote the excellent script, and Nicholas Hytner, the director, were very welcoming to me. In Birmingham, I was busier and had less time, but I gave a few notes, and talked to the cast about the way the characters develop.

“I like the simplicity with which Birmingham have had to stage the play — they haven’t got a revolving stage like the National, and can’t make it zoom upwards, and sink downwards. They’ve had to make the best of what they’ve got — basically a set of benches and tables, and that’s it. It’s very clever.”

lThe Birmingham REP production of His Dark Materials tours to the Oxford Playhouse (April 29-May 3). Tickets on 01865 305305 or online at www.oxfordplayhouse.com