Her manner is modern, even though she is over 80 years old, but her voice comes from an era of shooting parties and debutante balls.
Sitting by an open window in the Groucho Club in Soho, she eyes a plate of spring rolls and the braying members around her with equal interest. One of her many charms is a fascination with life in all its forms, a cast of mind one suspects she inherited from her mother, also a novelist, who embraced life with gusto.
When Weldon was 14 she was brought back to England from New Zealand when her mother divorced her doctor father, hence the housekeeper's job.
"I lived in the basement," she says, "while my mother struggled with upstairs, trying to keep her dignity in this extraordinary world which she wasn't used to. She was used to having servants. She was used to being on the other side." Weldon twinkles, which appears to be her default position, frequently accompanied by a flutey laugh. "On the other hand, she had always been on the servants' side. Her first memory, almost, was worrying because the cook only got half a day off a week. So she was a real born egalitarian and socialist, and remained so. So you could see it from both sides. You knew that the staff actually did have feelings, they did have emotions, they had actual lives."
Weldon has had a high-profile career as a novelist, a playwright and a scriptwriter, but not since she wrote the inaugural episode of Upstairs, Downstairs in 1971 – for which she won the Writers' Guild Award for Best British TV Series Script – has she drawn on that early and formative period. Until now, she has written very contemporary fiction, experimenting with voices and styles, and making a name for herself as a feisty, witty anatomist of society, as seen from a woman's perspective. Few who have read them will ever forget The Life And Loves Of A She-devil, Puffball or The Bulgari Connection, for which she drew fire for accepting money for product placement.
Flighty and flinty by turns, Weldon has ploughed a wholly original furrow through the literary world, attracting acclaim and disapproval in equal measure. Now, however, with her 30th novel, she has returned to the stage-set of a grand household. But while she has finally taken a conventional turn, the realism of her characters, her political eye and her irrepressible sense of mischief make for an entertaining, thoughtful read.
Habits Of The House, the first of a trilogy, is set in 1899, in the space of a few weeks. Written long before Downton Abbey aired, it was in part inspired by Weldon's childhood, but also by her interest in her grandfather Edgar Jepson's generation, around the turn of the century, which he strongly evoked in his novels.
It's the story of a rather dim-witted aristocrat, Lord Dilberne, who is facing financial ruin following an unwise investment in a mine in the Transvaal. Dilberne's spirited wife Isobel holds the reins of the house although, as Weldon shows, it is the staff who actually manage the show. Into this precarious scene wanders an American heiress who could be the answer to their troubles as long as their son Arthur can drag his attention from his mistress to this independent-minded young woman.
Habits Of The House, Weldon admits, "is just a romance" but it's filled with clever cameos of an age when revolution and rebellion were in the air. It was, she says, an "extraordinary time of intellectual ferment, which is always seen as a time when everyone was placid and smart and having fun. But it was not. It was a terrifying time. They thought the servants were going to slay them in their beds, they felt so guilty!"
When I comment that, despite some being loathsome, one has sympathy for all her characters, she laughs: "This I think is my feeling about people on the whole. One is not there to make judgments about them, simply to notice them."
Weldon's publisher has made much of the fact that she has taken an unusual turn with this book. As she talks, however, the reasons become more obvious.
"I just kept out of it, you see. I was in the novel so much usually, it just seemed I'd gone too far with the last one, because it was quite overtly about writing a novel, and the house I was living in. It's much easier to leave yourself out. It was really nice to be slightly disassociated from it." That novel was Kehua, inspired by Weldon's ghost-ridden house in Shaftesbury, where she lives with her third husband, Nick Fox, a musician and a poet who is also her manager.
Weldon's personal life has been as colourful as her professional career, so I ask why her autobiography Auto Da Fey stops once she became a writer. "It's just boring," she replies.
Really? That's not the impression one gets from the headlines. But when challenged, this mistress of the sweeping statement gracefully backtracks, covering her prints as if they'd never been made.
"No, because your life doesn't go on being boring. But in fact I have fed my life out in other novels and have more or less brought it up to date, which I think is why I've stopped, why I've done something different now.
"Actually, I think I've been feeding out the rest of my life through all these others – what is, I suppose, called metafiction. So there's always been certain people in the other novels who've been relating my life to date. I've brought it up to the end, more or less, so I'm now free, thank God! It just takes so many years!"
There's been much ground to cover, from her days as a single mother with a son, when she married a much older schoolteacher. He liked her to go off with other men, but didn't want sex with her himself. In fact, we are sitting in the district where she lived out this memorable episode, and where she also briefly worked as a nightclub hostess. Only a few years later, she was back in the neighbourhood, this time as an advertising copywriter, when she famously coined the phrase "Go to work on an egg". This was the same period as Mad Men, and she nods with recognition when she discusses the series. "I was just Peggy Olsen - It was completely accurate. It was great fun."
Did they really drink as much as the characters on TV?
"I do remember you would drink a lot at lunchtime. You'd be very drunk. And you'd go off to the Strand Palace Hotel, and spend an agreeable few hours there – it seemed perfectly normal – and then you'd go back to work." She sighs. "It's the energy you had which amazes me now."
Then there was her marriage to jazz musician Ron Weldon, with whom she had three further sons. The 36-year union ended when his therapist told him his star sign wasn't compatible with Weldon's (but apparently the therapist's was, and he went off with her). He died on the day their divorce papers came through.
One can see why it has taken so long to spin out the yarns. But there is another, more practical reason why Weldon has headed back into the past: "It has got almost impossible to grasp the present anyway, because everything changes so fast," she says, looking into the street as if it might have altered since we've been in the club. "Attitudes change, circumstances change. We're a completely different society than it was five years ago. But if you put it back in time it's easier to stop it and look at it at a certain point. You know what's going to happen. At the moment you simply cannot anticipate."
However, one thing she knows not to expect is to win any major literary prize. She says this without bitterness or conceitedness, and while some of her novels have been less than perfect – Weldon's most serious flaw is hurrying – her work is as good as many who have reached the shortlists and even the winners' podium. So why her lack of literary recognition?
She squirms, although whether with embarrassment or relish it is hard to tell.
"I never won the Booker Prize, and that is because in 1982 I quarreled dreadfully with the Booker people, who practically threw me out of their establishment."
At the time, Weldon was chair of the judges. "I was rather ignorant. First of all I gave the prize to Coetzee rather than Rushdie. They were very annoyed because he was overseas. And also because there was a speech you had to make, and I hadn't realised Booker printed the speech as part of their PR, and the speech was meant to be about the nobility of literature and the nobility of Booker for backing the prize, and their sensitivity to the arts."
She smiles, a little grimly. "Which, had I realised that, I probably would just have done, being a good girl. Except that it seemed to me that if you're going to make a speech, you'd better make a speech which meant something. And because the Society of Authors and the Writers' Guild were quarreling with the publishers who would not give us a minimum terms agreement and were behaving very badly towards writers – they still do – I made a speech saying so."
Weldon read her speech from an autocue, what she calls "those idiot boards". "You were making this anti-publisher speech and calling them to order. Well, usually in a speech like that, you say all of that first, and then in the last bit you say, 'But of course none of this applies to any of you here, who are all honourable, honourable people.'
"But, oddly enough, the unions, who I was supporting vigorously, were on a work to rule, and cut the power at 10 o'clock, and the boards went black, just before I got to the 'you are all honourable men' bit. And so because you are reading it, you don't know where you are, so I just sat down and said, 'That is really all I have to say.' So they didn't get that bit. And they were so incensed. And the head of the publishing union came over and hit my agent. It was a wonderful evening!"
She giggles, but it was far from funny or fair. "So after that I never got on to any shortlist. Well, one might not have anyway. But you can't help feeling that if you annoy the establishment too much, the establishment still exists and holds it against you."
To look at Weldon, writing ceaselessly as ever, and exuding joie de vivre, who cares about the establishment? One doubts they're having anything like as good a time.
Habits Of The House is published by Head Of Zeus, £14.99.