There are many reasons to be surprised by Prue Leith’s autobiography, from the celebrated businesswoman’s flirtation with sex, drugs and rock ’n’ roll to her secret affair with the husband of her mother’s best friend.

But the biggest surprise is why she waited until she was 72 to write it.

Relish is a great read; funny, honest and full of brilliant anecdotes about dispensing amphetamines to weary chefs, accidentally attending an orgy and doing battle in the boys’ club boardrooms of national corporations.

By her own admission, Leith has led an “incredibly lucky” life; born in South Africa into a happy family with enough money, she had a good education, started her own catering business aged 20 and then opened the Michelin-starred restaurant Leith’s, before adding her famous school of food and wine to her portfolio. By the time she sold the Leith’s group business in the ’90s, it was turning over £15m a year.

She has written countless cookbooks, launched several charity initiatives and is a former director of companies ranging from British Rail to Safeway. She went on a writing course once and was told she couldn’t possibly become a writer after such a blissful existence.

At that point, she adds happily, she’d already published two novels.

It isn’t perhaps surprising then that even her secret love affair with Rayne Kruger, an uncle figure who was her mother’s best friend’s husband, was followed by a very happy marriage that lasted until Kruger’s death in 2003. The couple even had his ex-wife’s blessing. Kruger was 39 and Leith 21 when they began their clandestine relationship.

“He had been like a godfather to me and while the relationship wasn’t actually incestuous, it wasn’t something anyone would approve of, including me.”

The affair was one of the reasons Leith dragged her heels about writing her memoir. “I knew I couldn’t airbrush out the most important person in my life but I didn’t want to hurt anyone.” In the end, she settled with sending proofs of the book to those involved and making amendments where necessary.

“I took a few bits and pieces out, especially at my children’s request.”

It would seem Leith’s son Daniel, a charity worker, and her adopted daughter Li-Da, a filmmaker, were a little squeamish about their mother’s enthusiastic descriptions of her very swinging Sixties.

“Daniel was particularly embarrassed when the Press first picked up on it all,” says Leith. “We had a little argument about it in fact and he later turned up at my birthday with an enormous present.

I thought, ‘Oh poor boy, he’s upset about this morning and it’s a peace offering. How sweet.’ I opened it and inside was a megaphone. ‘Just in case there’s a person left on the planet who hasn’t heard about your love life’.”

She laughs. But she surely anticipated the interest in an apparent bastion of respectability dropping acid and walking around naked at Parisian house parties?

“There’s been this enormous fuss about it as if it’s amazing…this was the ’60s! I did extremely little when you think of what some people did. Rock ’n’ roll – no; sex – yes, obviously but who hasn’t?; and drugs amounted to one disastrous trip and a few joints, which I think everyone’s tried.”

There was a sense of relief in finally having all aspects of her life out in the open, she says, not least her relationship with Kruger. She only recently realised he was the main reason she became such a successful businesswoman.

“Not just because he was such a good chairman and advisor but because he wasn’t there at night and wasn’t there much in the day. I only saw him three times a week and never for long, which meant I could concentrate on my business knowing I was loved and I loved.

"If I hadn’t had Rayne, I’d have been hunting for a lover, wanting to go to parties but I didn’t want to do any of that and I spent my weekends in London working, which seemed to me to be a great life.”

New love

When Kruger died in 2003, widowhood hit Leith hard. But a few years on, she had just got to the stage where “you think ‘God, it’s nice to have the bed to myself’” when, to her amazement, she fell in love a second time.

The musician and entrepreneur Sir Ernest Hall was an old friend she had known for years – then something changed. Leith says it was a revelation.

“I couldn’t believe that at the age of 66 I felt the same as I had the first time I’d fallen in love. That business of feeling sick, waiting for the phone to ring… it wasn’t any different. I assumed it would be some tame thing or about convenience or companionship and it wasn’t like that at all. So I’m now a great believer in geriatric lurve!”

She seems a happy sort, Leith. She’s currently enjoying a belated “gap year” she tells me – “a big mistake because I’ve now become interested in a hundred new things.” Aside from classical music, which Sir Ernest is educating her about, churches and stately homes, she’s become obsessed with auctions. “My cellar is full of all kinds of rubbish I don’t need but it’s just so much fun.”

I wonder if cooking remains as much a passion as it was? “I’ve never been an obsessive cook. I’ve always loved cooking and when I started getting grumpy, my mother would always say ‘What you need is to make a loaf of bread’. But I’m more driven by greed than perfection.

“There are chefs today who are wonderfully obsessed with every aspect of food – they’ll test dishes again and again. When I opened Leith’s – even when it got a Michelin star – we weren’t a patch on any of these modern restaurants. Food is a reason to sit down and talk for me.

I love to make big stews and cassoulet, which seems to me the ultimate family food. Me at the end of the table, doling out cassoulet with a ladle to family and friends – that’s my image of bliss.”