Royalty, Tara and best-selling books

Santa has her own shades of darkness, which surface occasionally, when, for example, she talks about Tara's rehab and how it changed the dynamic of her family. "We were a very English family," she says. "We were sent to boarding school at a young age and I loved it, but Tara didn't. Tara likes to talk about things but that's not my father's style. He's a farmer. He's off working on the land most of the time.

"My mother is more emotionally expressive and intellectual but sometimes I feel she has sort of martyred herself to my father's needs. Tara and I used to call their rather stiff and oldfashioned relationship 'playing drawing rooms'. But when extreme things happen in families sometimes it forces them to open up which can be good and bad at the same time."

When, I ask her, did her younger sister, the irrepressible It-girl and former cocaine addict Tara, turn out to be so problematic? "We are different people who do different things, yet people keep defining me as 'the sister of Tara'. Everyone wants to know what she's like and why she got so addicted to drugs.

When her cocaine problem was at its worst we were sharing a flat but I never saw her. She was out all night and asleep all day. I did use to confront her, but she always said she was fine. By the time she got really bad I was living with my husband."

Also extreme was a family skiing holiday with the Prince of Wales at Klosters in 1988, when they got stuck in an avalanche. The Prince's friend and royal equerry, Major Hugh Lindsay, died and Santa's mother Patti got buried so deeply in the snow it took the rest of the party, including the Prince of Wales, to dig her out.

"It was an awful shock," says Santa. "She was in hospital for months with two broken legs." That didn't stop Patti Palmer-Tomkinson from going back to Klosters. "Oh no," says Santa, "She's made of stern stuff, but she broke her leg again last week. I don't think she'll carry on now. It worries everyone so much."

Santa, who is named after her father's Senta barley crop, desperately wanted to be a writer when she left Exeter University, where she studied Italian and Spanish, but felt everyone would laugh at her. "For years I did retail jobs. I worked for Theo Fennell and even designed kitchens for Smallbone. Then I worked for Ralph Lauren and there was this witch of a woman who made my life hell.

"Inside I wanted to write. I'd jot stuff down in the evenings after work but never thought anyone would be interested in publishing me."

It was only after she met her husband, whom she calls Sebag, that her confidence grew. "Sebag is a great sounding board," she says. "He gives me confidence. He's really good at plots." She has always been terrified that people would only publish her because of her name. "I sent my first

manuscript out under the name Miss X. Three agents rejected it but one loved it." Her first book, Meet Me Under the Ombu Tree, was sold at auction in a two-book deal for £100,000. It sold more than two million copies.

She has already written her sixth, the Gypsy Madonna, set in France. "Sebag suggested I moved the settings of my books from South America to Europe," she says. "Apparently the Americans love books set in Europe. I don't know why."

Her husband's books are set in Russia. His latest was the bestselling Stalin. Quite a literary couple then? "I suppose so," she says. "We live in a flat in Battersea and it's a bit cramped.

We are going to move to a big house in Notting Hill but, for the time being, Sebag and I share a study, so he's there writing about Beria or whoever and I'm trying to describe romances in war-torn France. We argue a lot because I like to have music on that reflects the mood of my novel, and he hates that." The rest of the time is taken up looking after their children, Lily, four, and Sasha, nearly two.

"Sebag adores them," she says. "He'd never leave them. They look just like him, thank goodness, so they are all totally imprinted on each other. He's a dreadful wanderer is Sebag but I'm very laid back about that stuff. I don't have a possessive bone in my body. You know, Sebag's the most dreadful flirt. He loves women. He has so many girlfriends.

"I could never have married one of those boys I played tennis with. They were so normal. Part of me has always wanted someone more exciting, more intellectual. Sebag can't play tennis. He hates sport apart from boxing. He's got loads of books. I've always loved that about him."

Before they married, Santa converted to Judaism, her husband's faith. It took her a year of learning Hebrew and the scriptures until, eventually, she was given the all-clear, as it were. Why did she do it? "Sebag didn't ask me to, but his faith is very important to him and I wanted to share it. I've always found the Jewish faith fascinating. I really embrace it. But I'm no good at making chicken soup. You're not really Jewish until you can make good chicken soup, are you?"

The couple's shared faith has, no doubt, brought them even closer together but there are hints of insecurity. "He goes away a lot to research," she says. "I never really know what he's doing but he always comes home pretty quickly."

The rest of the time they spend together, writing, boxercising with their personal trainer, or popping in and out of society parties. "We get on so well," says Santa.

"When I was in my early twenties I went out with an Argentinian computer analyst who was a decade older than me. I went backwards and forwards from Buenos Aires. I loved him and would have married him if he had asked. It took me ages to realise he was a controlling bore who had undermined my confidence. He told me what to wear and what wine to drink. When I met Sebag, I knew we would never do that to each other.

"I had to work very hard to get him to commit to me. I did what my mother told me, like that book The Rules. I didn't return his calls sometimes. I only talked on the phone for two minutes. I was cheery and confident. Now we are married, so it all worked out. Mind you, I'm sure some family friends were appalled. Sebag is probably not seen as being all that 'proper', in fact, not 'proper' at all!"

But that is, oddly enough, how the very proper Santa Sebag Montefiore prefers it.

The Last Voyage of the Valentina is published on 10 March by Hodder and Stoughton, price £14.99.