She is fiercely proud of her husband, who has resurrected his career when many had written him off. "That was not a tragedy. Not in terms of real life experiences. And it has turned out to be a triumph for Maurice, a total triumph," she gushes. "He has been extremely brave and has been joined by people whose loyalty and self-sacrifice has been extraordinary, and he has had the greatest start to a new agency in the history of advertising."
Josephine Hart is nothing if not effusive. Given the chance she would go on and on about her beloved M, as she calls her husband. And she goes on and on about sex and death. She is also effusive about her native Ireland. She has been chair of the Ireland Fund of Great Britain for seven years, and resigns - on a high about the peace process - after its ball on Monday.
Maurice, she admits, has been a powerful influence: it was he who apparently set her to writing, in her forties, by locking her in a room and making her write for two hours. The theme of her early novels, Damage and Sin, was obsessive love: the new book, Oblivion, reprises the theme, this time from beyond the grave.
"My imagination, clearly, is very dark," she says, without embarrassment. "I am absorbed by sex and death, the beginning and end of the journey."
"Sex and shopping" novels set in the world that, stereotypically, a rich advertising mogul's wife should inhabit, hold no interest for her. Dead poets (Eliot, Auden, Yeats) interest her more than living designers ("My greatest treat," she enthuses, "is to have duplicate copies of my favourite poetry books in each house").
The Saatchis' homes (in Mayfair, Sussex and France) bear the stamp of M's fiercely stylised taste - lots of pale stone and marble, white furniture and neo-classical urns and statuary. They are both museum and mausoleum, with carved flowers and sculpted trees that will never wilt, and white arum lilies, so often a tribute to the dead.
"M is usually right when it comes to how things will look. He has a wonderful eye. I don't notice the physical things around me much, so M is the person who makes all the decisions about how the house should look, but we both share a passion for order, and like calm, neutral colours."
Did Maurice losing control of his company disrupt this ordered world? "I've never thought of these issues as life and death. I've always had a different perspective on everything to do with worldly success. Of course I'm a human being, and when I get a bad review I don't like it, but it has no effect on my life."
Didn't they consider telling the world to go to hell and enjoying the freedom they can well afford? Hart laughs off the thought. They are both too motivated to step off the ladder. She is not, she insists, motivated by success. "I don't believe in being obsessive about 'literary aerobics' every morning, but I became quite obsessed with this book in a way I didn't with the others. I would get up and start working at 5am, break for a swim, and keep working, fanatically."
She says her books were all written inside her head: "I already know what my next one, and the one after that, will be." The thought of going to the US again on a plane this autumn to promote her book there fills her with terror: she hates travel of any kind. In some ways at least she is still a small town convent girl.
And like a true convent girl, she obsesses about death. "It's hard to have a serious discussion of death," she says earnestly. "It's considered bad manners. Sometimes people are so frightened of the journey towards death that they fill their lives and their homes with the minutiae of surface things.
"I've known since I was a very young girl where the journey ends," she continues portentously. "I also know that the landscape of grief can be a very seductive place, and that the rage of grief can drive perfectly wonderful people to the thought of murder in a second. I lost three of my siblings before I was 17 - two of them within six months of each other, my sister from an illness, my brother in an accident. If that happens to you, you begin to consider death on a daily basis. For me that's been wonderfully enriching, because you see things in context - the way some people only do when they find out they're terribly ill, and realise they didn't know what life was about."
She still owns her childhood home in Ireland. "I can't let it go, can't let it, and don't know what to do with it. It's a problem. I'll have to deal with it someday."
Have she and M talked about how they'd like to go when their time comes? "I know I wouldn't like to be cremated," she says slowly, "because I think the body of someone you love is terribly important, even in death - I can understand why people are begging the (sic) Sinn Fein to tell them where the 'bog jobs' are, for a proper burial.
"I often talk to M about that, although I haven't made specific funeral plans. I think I'd like some Eliot read. M would know what I'd want. He would know. I know that if something happened to me everyone would, quite correctly, guide M and the children into a future that doesn't hold me."