There’s nothing like winning the Nobel Prize for Literature to cheer up a household. In the last few years, Chez Pinter-Fraser, the West London home of Harold, the edgy playwright, and Lady Antonia, the elegant historian, must have had to fight off the gloom. In 2002 Lady Antonia’s father, Lord Longford, died and, not long afterwards, her mother. The same year, Pinter contracted the cancer that killed John Thaw. He recovered but his health has recently again been poor. A rare auto-immune disease attacked his mouth, threatening to render speechless a writer famous for his theatrical pauses. And then, three Mondays ago, returning from a festival in Dublin that marked his 75th birthday, he slipped getting out of a car, smashed his head on the pavement and was taken by ambulance to hospital.

“He looked,” says Lady Antonia, “like he was gone.”

Unconscious? “Not quite, but with the head you never know. And I had the macabre experience of filling out forms and every time I put his birth date down everyone would say, ‘But it is his birthday!’ and I began saying, ‘Yes, what a birthday!’ but by the end of it I thought about forging a different date so I didn’t have to go through it all again. And then we came back and he was, naturally, after nine stitches, feeling terrible and we were just sort of recovering and this wonderful thing happened.”

The phone call? “Yes. Twenty minutes before the official announcement.”

And suddenly he didn’t feel so ill? “Quite, no. He felt very jolly.”

A jolly Harold Pinter would be a welcome sight but he is in his garden study, waiting for steroid medicines to heal his mouth and pondering whom to lambaste in his Nobel acceptance speech in December. As in an old fashioned play, this morning’s interview is a one-set affair, confined to the drawing room, with noises off and Harold ringing from time to time on the internal phone. Still, I sense his euphoria. Everyone has been terribly kind. Their Portuguese housekeeper said the news was all over Iberian television. Fraser’s oldest granddaughter, Stella, rang from Yale to say people were coming up and congratulating her.

What, I ask, will he do with the money? The present krona-sterling exchange rate makes it worth around £730,000. “Harold said that I should buy myself a dress for the banquet. He said the upper limit was £700,000 and he would keep the rest.”

I thought she hated clothes shopping? “I don’t like it, but this afternoon I’ve told myself I am going to go and get a dress.”

When the good news came, I confess I had to stop the chippy side of me wondering what need Lady Antonia had of more glory, albeit, in this case, reflected. To be born a Longford is, surely, to be pre-awarded one of life’s glittering prizes (although, in fact, the courtesy title “Lady” came only when her father was upgraded from Baron Pakenham to the Seventh Earl of Longford in 1961). She has gone on to live a most privileged life, blessed with two rich husbands, six children and 16 grandchildren. With Pinter, she forms an unassailable position in British society, but even without him she would be fabulously well connected. In our hour’s conversation she artlessly cites as pals Barbara Walters, the first lady of American TV, Marigold Johnson, Paul Johnson’s wife and London society’s leading lay Roman Catholic, and the television director Tristram Powell, whose father, the novelist Anthony, was an uncle. Edward Fitzgerald, QC, “ the leading human rights barrister”, is, she mentions, a son-in-law.

Just as, however, her large and enviable house is less tidy and stately than you’d expect — her study upstairs is apparently a real mess — it takes only a little imagination to appreciate that her 73 years have not all been golden and contained both sadness and hard work. Like her CBE, the prizes she has won for her readable but diligent history books have come on merit. Indeed, her thrilling retelling of the Gunpowder Plot, which won a Gold Dagger Award in 1995 and has just been reissued for the plot’s 400th anniversary, proves to be both astute on the players’ characters and moving on the Catholics plight in paranoid Jacobean England.

She even wonders now if she was too fair to the conspirators. “I was re-reading it after 7/7 and 21/7. I thought I would not now have used the word ‘misguided’ on the last page. I think ‘brave bad men’, which I also say, is the correct judgment. I think that their appalling suffering at the hands of the government, the torture and those frightful details of execution, in some way softened me.”

Would she still have ended with a quote from Nelson Mandela’s speech at the Rivonia Trial? “I would always, because I think the point is valid. Anthony Howard dared attack my use of the quote in 1996 which I thought showed he’d missed the point of the whole book really.” But doesn’t it taint them with innocence by association? “Oh, I didn’t mean to make them innocent. I just meant to say that as long as you persecute people you will actually throw up terrorism. That’s what I meant.”

As in several of her other histories, including Mary Queen of Scots, which brought her renown back in 1969, and Marie Antoinette: The Journey, currently being made into a Sofia Coppola movie, Fraser’s narrative is permeated with her understanding of Roman Catholic sensibility. She comes from a family of converts. Her father, the first, she thinks, took to Catholicism as a way out of “great crisis” after a nervous breakdown at the start of his war service.