With her siblings, she built camps in the woods, went horse riding and swimming in the river, played tennis and croquet, and explored every nook and cranny of this green and pleasant land near the village of Dummer.
We drive up to the recently refurbished Gardener's Cottage - big garden, big cottage, although she insists it's small - on her aristocratic parents' 1,200-acre estate. Montefiore and her family retreat here from their London home every weekend. The elder daughter of an old English farming family, her father is the former Olympic skier Charles Palmer-Tomkinson and her mother, Patty, is a glamorous Anglo-Argentinian, best friends with Camilla, Duchess of Cornwall.
The 37-year-old writer is also good friends with the Prince of Wales - her father was Charles's ski instructor - and the Princes William and Harry, as well as Kate Middleton and sister Pippa, are chums, too.
Her little sister is the more eccentric former It-girl Tara Palmer-Tomkinson, and they have an older brother, James, a banker. Therefore, given the media's fascination with Tara's lively activities, her well- documented drug problems and the family's royal connections, they are rarely out of the gossip columns.
"It's just something you learn to live with," sighs Montefiore, whose full name is actually Santa Sebag Montefiore. She married the high-society, buccaneering hack-turned-historian Simon Sebag Montefiore in 1998, in what the Times described as "a highly-hyphenated marriage". She dropped the Sebag because it was too much of a mouthful for the dust-jackets of her sub-Mary Wesley-meets-Jilly Cooper novels.
"It's bad enough having a name like Santa - I'm named after a crop of barley that my father produced called 'senter', and my mother compromised with Santa, which means 'saint' in Spanish - although nowadays there are so many odd names around, mine seems much less strange.
"As a child, I hated it and wanted to be called Jane. Now I enjoy the fact that nobody else has it," she says.
Since the late 1990s, when she published her first best-seller, Meet Me Under the Ombu Tree, she has written six more beach-read blockbusters, which have been translated into 20 languages and sold more than two million copies.
She describes herself as "an old-fashioned" writer. "That's why I write so much about the past. I love the Fifties, for instance - the fashions, the morality of the time; I just feel that's my era".
With the tea and coffee made, 42-year-old Sebag or "Bag" - as she always refers to her husband - is instructed to take their children, Lily (six) and Sasha (four), outside to play in the garden while we repair to his study.
They share this room at weekends. He sits with his back to her at his remarkably tidy desk while she perches on the large red sofa. Before they moved to their Kensington home they used to live in a flat in Battersea and they would write at opposite ends of the dining-room table. There were frequent arguments about the choice of background music - she likes soothing stuff by Ennio Morricone and he has a preference for Guns 'n' Roses and David Bowie. "It's surprising our marriage survived, because I wrote five novels while Sebag was writing about Catherine and Potemkin at the same table."
None the less, Sebag praises his wife in the acknowledgments of his last book, Young Stalin, for her "sunny encouragement and serene charm". Meanwhile, she insists she would never have begun writing in earnest had it not been for his conviction. "He's given me a lot of confidence, real belief in myself and my abilities. One of these days we might write a children's book together. Someone recently suggested we do an epic novel - he'd do the history and I'd do the 'fluffy bits'."
Between them, the Sebag Montefiores have quite a literary cottage industry going, although she writes only when the children are at school. "I didn't want to hand them over to a nanny. My mother was always there for me," she says.
The paperback version of her seventh novel, Sea of Lost Love, was published last month, The French Gardener comes out in March and she's currently at work on her ninth. Which would seem to suggest that, unlike her sister, she is not much of a party girl. "Part of me would really love to move back to the country," she admits, "but Sebag is essentially urban and we do both like living in London so long as we have the cottage to escape to."
Her husband is incredibly flirtatious, she laughs, adding that it has been written they have an open marriage. "We don't!"
His previous girlfriends include Koo Stark and fashion writer Nicola Formby. They met when one of Santa's failed suitors suggested to Sebag he check out "this amazing girl". At the time she was working in Santa Maria Novella, a perfume shop on London's Walton Street (she has also done PR for Ralph Lauren and society jeweller Theo Fennell).
Sebag did check her out and found her "celestially beautiful, so pure, so romantic, so wholesome and so unusual". She says she doesn't remember him, but six months later he called and invited her to tea. "I fell in love with his voice," she says, her cheeks flushing. "I was terrified when we met in case he was ugly, but he was wonderful. I'd never met someone so unusual, so entertaining or so eccentric.
"I knew I wanted to marry him on our first date. Over tea, we found we both knew the musical Evita by heart and we ended up singing it from beginning to end. Sebag says he fell in love with me when I played the piano.
"But I also played hard to get; I read that American book The Rules, about how to trap a man, and I played by them. Although I was madly in love with him, at first I didn't return his calls. I had to treat him as if I didn't fancy him at all - and it worked," she giggles.
Their wedding was at the Liberal Jewish Synagogue in St John's Wood and was attended by Prince Charles and the then Camilla Parker Bowles - their first public outing together after the death of Diana, Princess of Wales. They partied afterwards at the Ritz.
Did she mind the fact that all the flashbulbs were popping for Charles and Camilla? "Not at all - my wedding photograph ended up on lots of front pages; I was delighted they came to my wedding. It was the happiest day of my life - I was just so thrilled to be marrying Sebag. That was all I could think about. In any case, the ceremony was totally private and that was what mattered to us."
In order to marry Sebag, Santa, brought up an Anglican, learnt Hebrew and converted to Judaism, which she says has enriched her life enormously.
"One of the nicest things about the Jewish religion is the fact that it is practised in the home - every Friday evening we have candles, a Jewish dinner party. We celebrate Hanukkah with Simon's parents and we come down here to spend Christmas Day with my family. We exchange presents but we explain to the children that this is not their festival. Sebag didn't insist that I convert, but he wanted me to understand Judaism and to understand him. I'm so happy that I've done it. When we went to Israel, I wasn't just Tara's sister. I was a Montefiore - they're such a respected family there," she says.
The Montefiores are one of the great banking families of Europe. They came to England from Livorno, Italy, in 1800, and by the time Sir Moses Sebag Montefiore, Sebag's great great uncle, died he was one of the most famous Jewish men in the world. A friend of Disraeli, Palmerston and Queen Victoria, he had visited the tsars and was in business with Nathan Rothschild, to whom he was related by marriage. His image is still on bank notes in Israel.
"For both of us family history and traditions are very, very important," she says. "We have the most wonderful extended family life and closeness, with his Jewishness, my father's roots and my mother's Anglo-Argentinian heritage.
"Both Tara and I were sent to boarding school at a young age - she's two years younger than me. I absolutely loved it, but she loathed it. She feels things very deeply, but I believe very much in reincarnation and I think Tara is a very new soul, so life's very difficult and much harder for her in a way it isn't for me. She's very dear and we're close. She's happy careerwise at the moment, saying, 'I don't need a man'. But I would love her to have someone to share her life with; I hope she will one day.
"We're all always there for her. She's the most fantastic aunt to my children - she gives fabulous presents. But I thought she was mad when she went into the jungle for I'm a Celebrity... , but then I guess she is a little mad. As for her performance in Fame Academy, we were so proud of her. In the middle of Sainsbury's my son got down and put one leg out and started growling: These Boots are Made for Walking - just like Tara," she says, giving me a demonstration.
At the same time, though, Santa clearly needed to establish a little distance from her sibling. "Of course she's had her problems, but I can't go around being defined as 'the sister of Tara Palmer- Tomkinson' and having her words put into my mouth - which has happened.
"My family name is distinguished and it's been useful, but I was glad to be free of it. At school, she was my little sister, then to the press I became only her sister. I didn't want to go on being somebody's daughter; then someone's sister.
"I wanted my own identity. Falling in love with Sebag and changing my name to Montefiore has been the most wonderful thing. I've finally become my own person; it's such a relief."
Sea of Lost Love by Santa Montefiore is published in paperback by Hodder, priced 6.99.