James Runcie describes his faith as “a bit off and on” making him an odd choice as headline speaker at the Bloxham Festival of Faith and Literature this weekend.

Granted he writes the globally successful Grantchester novels, is commissioning editor for Radio 4 and the son of Robert Runcie, former Archbishop of Canterbury, but his own religious beliefs are nowhere near as fervent as those of his father’s.

Or as he explained it: “I met a friend of my parents recently who said: ‘It’s your fault. If you will keep writing about a vicar solving crimes, what do you expect?’ So it’s inescapable really.”

Instead, James Runcie escaped to a tiny village near Fife in Scotland with his wife and children, as far away from the ecclesiastical centres of his youth as possible.

“People think I live in Cambridge because that’s what I write about, but haven’t they heard of using one’s imagination? You can make up fiction,” he says laughing, before acceding that having been a student at Cambridge and growing up surrounded by the clergy, he had quite a lot of material to plunder.

His Scottish home may be ‘incredibly inconvenient’ for his weekly trip to London’s BBC but it affords him the space required to inspire his famous Cambridge detective, and also, one suspects, to be himself.

But then James Runcie readily admits to trying to escape from his father’s reputation and expectations, despite the endless accolades continually pouring on his own famous shoulders.

“When I was growing up there was always a little picture of me and a big picture of my father in the papers, and now there’s a little picture of me and a big picture of James Nolan instead,” he jokes. “So there will always be people who give me a look and think: ‘Yes he’s interesting but he’s not as good as his dad.’

“But then I’m not a clergyman and I don’t behave like one.”

And yet here he is as guest speaker at the Bloxham dinner on Friday night with a full talk on Saturday morning.

“I tend to avoid talking about faith, and think Thomas Carlyle summed it up best when he said his was “a life of doubt enriched by faith,” so I’ll defend religion, but not profess it. Although of course that’s not always possible – a recent interview with the Salvation Army started off with : ‘So how would you describe your relationship with Jesus?’” he chuckles.

So why put himself through a festival of faith? “Because Bloxham is so much more thoughtful than other literary festivals.

“It’s not just endless authors trying to flog their latest books, it has themes and philosophical discussions so that the audiences can join in and really get stuck in. It’s more about the morality of being alive.

“And besides, there is this current debate about how festivals have replaced churches, that they are secular assemblies in themselves where people can gather and talk about things, that they are full of people trying to find the spiritual in the everyday, so it all locks into that.

“Besides, I love coming back to Oxfordshire.

“It’s still the one place where I don’t need a map because I was brought up in Cuddesdon near Wheatley and went to The Dragon School, so it always feels like I’m coming home.”

Endlessly prolific in his output, from his Radio 4 hat to documentary making, novel writing, TV producing and as a playwright,with the Grantchester prequel done, what is he up to? “A novel about Bach and his St Matthew’s Passion,” of course.

As for Granchester, next year’s TV prequel is set in the Second World War, and addresses how and why James’s literary hero becomes a priest.

“It is something I have always skated over until now, so it was quite an emotional journey for me, about faith and violence and a commitment to the priesthood. Yes, a big leap and all subject matter I can relate to.”

His family friend had a point then.

Bloxham Festival of Faith and Literature

Friday - Sunday

bloxhamfaithandliterature.hymnsam.co.uk