When I tell Professor Andrew Roberts — historian, BBC presenter and best friend of David Cameron — that we’ve met before he looks aghast. “Christ, we didn’t sleep together, did we?” A hand leaps to his forehead. “Did I make some drunken pass at you in a darkened room at some nightclub?” I assure him it was just a handshake at the launch of one of his books (he’s written 12).

“Thank Christ. I was a totally different person back then, that’s the thing.” He takes a slurp of beer. It’s midday and we’re at the Belgraves Hotel, a swanky number with a branch in Brooklyn, just aound the corner from his Knightsbridge home (“the house is being refurbished so we’re renting a little place”).

By “back then” he’s referring to his single days, when was so prolifically present at parties that he was nicknamed “the socialite historian”. An Observer profile accused him, “with his successful romantic conquests”, of rivalling Casanova.

He’s keen to kill this reputation now he’s turned 52, and impresses on me how dull he is. He’s just returned from a visiting professorship at Cornell University in New York with his wife Susan Gilchrist, who is the  global CEO at Brunswick Group, and has recently taken up a post as visiting professor of war studies at King’s College London.

“I’m actually really enjoying the serious and substantial stuff like writing big books and appearing on TV shows,” he says. Last night he was invited to three parties “and didn’t go to any of them”. He doesn’t miss the carousing of the old days and can only explain his former habit as “some sort of psychological desire to be liked. There’s an age for everything and it’s like a book, you turn over a new leaf and start a new chapter. I’m more mature now.” ...