She has always been a keen observer of relationships. She remembers her own father coming back from fighting in Egypt after the Second World War, the Union Jack hung out to welcome him. "I think my mother's generation had a very difficult time in their marriages because of the long periods of absences and separation."
But she says she wouldn't dream of offering advice to her own children without being asked. "It is sometimes agonising to watch when things go wrong, but that's the whole thing about the book. It's to say that arguments and rows don't mean that a marriage is on the rocks."
The Fearnley-Whittingstalls married in July 1962 in Henley-on-Thames, where Jane's parents had settled after returning from what was then Rhodesia. In those days, she says, weddings were usually in the afternoon, and the reception tea consisted of cucumber sandwiches, strawberries and cream, and a choice of tea or champagne.
After staying at home to bring up Sophy and Hugh, she embarked on a new career (encouraged by her husband Rob, an advertising copywriter) and trained to be a landscape architect. The idea for the book came from looking at the relationships of couples who had created gardens together – Harold Nicolson and Vita Sackville-West, Rosemary and David Verey.
"I just got a flash," she says, "that there was a much broader issue here – marriage in general and how couples manage to live together." And there is, she says smiling, a link between gardening and marriage: in both cases, you have to be patient and nurturing, and be prepared to wait for results.
If Jane is an ambassador for marriage, she's also a realist. She knows, as she says in the book, that the promises we repeat when we stand at the altar are easier to make than to keep. But, as she says, "I would hope that people with strong, long-term relationships can somehow pass on whatever wisdom they have gained. Which, I suppose, in a small way, I'm trying to do – blow the trumpet for marriage a bit."