Let’s begin as they would on Through the Keyhole. Here is an ordinary room, a study, in a central London penthouse. What clues can we gather about the owner?

Well, on the desk are two biographies of Sir Alex Ferguson. So, probably a football fan. Some of the bookcases are lined with videos, so possibly somebody who works in television. On the wall over the fireplace, there is a photograph of Nelson Mandela, with David Frost. And here’s a picture of George Bush Sr, also with Frost.

On the way, you’ll have noticed a picture of Bill Clinton and Tony Blair, with Frost, and another of Vladimir Putin, with Frost. Any ideas yet? If not, take a look at this comfortable armchair. Note particularly the man in the blue suit, sitting in it and pouring himself a second cup of coffee. Right first time. Our special guest today is, of course, Sir David Frost, the host of Through the Keyhole.

It is now 25 years since a breakfast television producer called Kevin Sim thought it would be fun to visit the homes of famous people and poke fun at their knick-knacks. It ran as a slot on TV-am for a while, but was turned into a series in 1987. Last week, it returned for another run on BBC2.

Lisa Snowdon has taken on the role of snooping around the houses so long associated with Loyd Grossman, while the guests for this series include the model Caprice and the hairdresser Nicky Clarke.

“It’s fascinating to see people’s houses and to psychoanalyse the homeowner,” Frost says with his customary enthusiasm. Quite so. Yet I cannot help noticing that the host of Through the Keyhole was reluctant to allow me into his own home. We are speaking instead in his office in Kensington. He says it’s very much like his “den” at home — lined with books, the walls a vivid blue, piles of paper everywhere — but it’s hardly the same, is it, David?

“One day, I shall probably relent because the people who do Through the Keyhole really enjoy it,” he says.

“I suppose, in my case, it’s cherishing privacy. Lots of one’s life is public, and that’s fine, but it’s nice to have a bit that isn’t.” Billy Connolly put it slightly more brutally. After a personal approach to come on the programme, the comedian wrote back: “Dear David, on the whole, I’d rather be burgled.”

The first guests were Stirling Moss and Spike Milligan, whose home proved something of a surprise. “We expected it to be all zany and wacko,” says Frost, 69, “but it was an immaculate Victorian residence with nothing out of place.”

Not every house is what it seems. Last year, the series toured a penthouse flat belonging to the actress Antonia Okonma, of ITV’s Bad Girls. Nobody guessed, but that was no surprise — unbeknown to the producers, she actually lived in a modest council flat with her parents. The penthouse belonged to a friend.

So, why do people agree to throw their homes open to the viewing public? After all, part of the treat for the audience is hearing members of the panel poking fun at the interior design. The late Alan Coren said of a soap actor, whose name now escapes Frost, that his sparse flat “looked like a terrorist safe house”.

“About half of the people take the Billy Connolly view,” Frost says. “The others are proud of their house and want to share it with people.” So, if we were to go through his own keyhole, what would we find?

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