CONT. "Oswald Mosley had an affair with his dead wife's sister," I venture, tentatively.

"With both of Cynthia's sisters I think," Diana informs me.

"This must have been very shocking for you."

"Well, not really. I think it's very common."

"But you were still young - he was your new love - you surely found it painful."

"Only marginally. I think if you're going to mind infidelity, you better call it a day as far as marriage goes. Because who has ever remained faithful? I mean, they don't. There's passion and that's it."

"You're obviously not a jealous person."

"Not very, no. I might be jealous of a deep friendship, something like that. But not sexually jealous. Kit and Baba always had this thing for each other, and it's life. And with sex, opportunity is so important."

"There's always plenty of opportunity!"

"No, there isn't. Not always."

"Did you have any amours after Sir Oswald?"

"Ah, well, like Wilde I can resist anything except temptation - but I was never in the slightest degree tempted."

Lunch has begun, served by the maid at a table at the other end of the drawing-room. Diana had said: "I invited Jean-Noël to join us. I hope you don't mind, because my hearing is so bad and he can help." Which he has been doing, sometimes by shouting what I've just said or by writing it down. Jean-Noël is in his thirties with thick black hair which flows upwards. Later on he says: "Diana is my best friend. I visit her three or four times a week."

The first course was tomato and mozzarella salad and now we're tucking into roast chicken with vegetables. Well, I am - the other two eat very modestly. There's a feature I haven't seen on a private lunch table for many years: finger bowls, in emerald glass. Beneath the table is a smart rug of black and white diamonds.

"What is your favourite thing in this room?"

"My clock and barometer." She indicates the French gilt pair hanging on the wall opposite the windows. "They belonged to my great great grandfather really - but I bought them at one of my father's many sales. He was always having to sell things and always at the bottom of the market."

The voice is not plummy, is not the Oxford or Bloomsbury drawl, but the perky cut-glass deb voice of the 1920s and 1930s. It is very clear, and she has almost flawless grammar besides. Cheese - Diana doesn't have any - and green avocado salad are followed by a superb chocolate flake with lozenges of gold leaf on top. After coffee we decamp to the sofa.

"There's a new book saying Hitler was homosexual."

"I'm sure he was not homosexual - because that sort of thing I do more or less understand." (Diana has always had gay friends, from Lytton Strachey onwards.) "With someone like General Montgomery - it may well have been unconscious - but all the ADCs and other people around him were very good-looking young men. And I believe it was the same with Kitchener. Well, now, Hitler's adjutants were sort of ..."

"Ugly."

"Gnarled old men, they really were. They were very, very sweet but I'm afraid not the least bit good-looking. That just is the answer really, these were the people Hitler loved being with."

"It's widely accepted now that his relationship with Eva Braun wasn't sexual either."

"One can't be utterly sure about anyone - except oneself. But I don't think sex was a big appetite in him."

"Which is strange. Because very powerful men are usually very sexual too. Was he like a eunuch?"

"Like a eunuch? No, but, well, there was no question of anything between him and me but, you know, one can still feel it - and with Hitler one couldn't."