Peter Capaldi is striding around the Tardis set in the Cardiff studio, fretting like a host whose oven isn’t working.

“I can’t turn the lights on because we don’t have any electricians here today,” he says, apologising for the lack of technical support that means we are peering at each other in semidarkness inside television’s most famous time machine.

“So you don’t really get a sense of what fun the Tardis is. It’s quite cozy. Quite wombish and warm. People are always happy to film here. It’s not like being in Midsomer Murders, is it? There isn’t another programme in Britain in which you get to roam around 21st-century Venus.”

At the age of 58, the incumbent Doctor brought something quite new to the role, an element that the younger Who fan wouldn’t even remember, growing up with David Tennant and Matt Smith – a sober, grownup demeanour that some people might call grumpiness.

When he talks about his Who, Capaldi is anything but cranky, always almost-smiling, on the brink of some joke that he sometimes tells and sometimes chooses not to.

How does it feels to be an old Who after a string of young ones? “Surely you mean older? The Doctors I grew up with were not young. To me, Doctor Who was not a young man. Ironically, I’m older than any of those, except for William Hartnell.”

William Hartnell, the first Doctor

The Tardis itself hasn’t changed very much since Matt Smith’s time. There’s a stack of Marshall amps, a nod to the idiosyncrasy of Capaldi’s Doctor being madly into guitars, and the odd piece of 60s furniture, but “I think it cost so much that it would have been a poor use of taxpayers’ money to get rid of Matt’s and replace it with mine”.