It has been a good decade for Mary Wesley. The best-selling author of The Camomile Lawn, who wrote her first book aged 70, has sold 1.5million books in paperback. ‘I’m always terrified when I’ve finished a book that it’ll be a disaster,’ she says. ‘I used to think, ‘What am I trying to do? I can’t write.’ Now I still think each book will be my last.’

Her books have been described as like Jane Austen with sex: witty and racy, they are well-mannered English novels about upper middle-class people with a workaday knowledge of adultery and illegitimacy. They line one shelf (including Japanese and Danish translations) in her cosy cottage in Totnes, Devon.

This week sees the publication in paperback of A Dubious Legacy (for which film discussions are under way), and Harnessing Peacocks will be on television this spring. ‘I’m really pleased with Harnessing Peacocks,’ she says emphatically. She advised the director and went on set for a couple of days – ‘terribly boring and repetitive’ – but once they were filming ‘wouldn’t have dreamed of interfering’.

Wesley, a spritely 80-year-old, is elegant in her favourite black cashmere jumper and trousers. She is weeny, with silver hair and a face that expresses strength and timidity. Seemingly brisk, tough and upper-class on the phone (it transpires she doesn’t like talking during ‘office’ hours), she now seems mischievous and her acerbic intelligence shines through. She is remarkable for her age.

‘People talk about me writing sexy books. They only say it because I’m old. Absolute tripe!’ she chuckles, sardonically. ‘People think sex is something you give up, but the instinct goes on.’ Inside, she says, she still feels about 30. ‘I had the menopause, which wasn’t particularly bad. I’d love to have sex now. But it isn’t there, so I exist without it. I think it’s very important. It makes you feel good, doesn’t it?’

She says she was educated by her lovers – having received no formal schooling, just 10 governesses in eight years. ‘I had clever, intellectual lovers who encouraged me to read,’ she says, belying her reputation for being secretive. ‘They were amazed at how ignorant I was. It was rather upsetting really.’ How many lovers has she had? ‘That’s rather a close question. I don’t think I should count them.’ She hoots with laughter. ‘I’m sure the few who are still alive would hate to be listed!’

She was faithful to Eric Siepmann, her second husband and ‘true love’ who died of Parkinson’s disease in 1970. ‘I still feel sad. I miss him the whole time.’ Would she ever like to have another man in her life? ‘When Eric died, I thought about it. But I realised I couldn’t go through that all again. We had 25 years together, and there was really nothing left to give. You put so much into a relationship. It was wonderful and anything else would seem a sham.’ She felt cleaved in half when he died. ‘It took a long time to adjust to living alone.’

The characters she writes about are now her company. ‘Rather a good sort of company,’ she laughs. ‘You can kill them off if they bore you.’ But she’s not lonely. ‘I’m solitary by nature.’ She protects her privacy savagely, sometimes going for five days without talking to anybody. ‘As a child, I would hide behind a tree to avoid speaking to somebody.’

Wesley has had a life that she could excavate endlessly for her fiction. Born Mary Wellesley, the daughter of a colonel, she is descended from the Duke of Wellington’s eldest brother and had a peripatetic, unhappy and solitary childhood, though she says she didn’t feel noticeably alone at the time. Did she feel loved? ‘No,’ she says, pensively. ‘I never felt loved by my mother and I hardly saw anything of my father. We were always moving house, so I never had any friends of my own generation, and I didn’t go to school. I was frightened of other girls – I always thought they were very sophisticated and confident.

‘I was probably a great disappointment to my parents. I didn’t grow up to be conventional in the way they wanted. They were much happier once I left the nest because my mother wanted my father to herself.’

Her first marriage was largely spurred by the desire to escape her family. It was short-lived. During the war she worked in intelligence, and for seven years during the Forties she lived ‘in sin’ with Eric, a journalist. Finally they married and when he died they were living impecuniously on social security. She started writing in her thirties, for herself, famously binning her efforts. She has three sons (two from the failed first marriage) and is a committed socialist.

Her childhood has left its legacy of self-doubt. She perceives herself as shy and hesitant. ‘I think I’m getting better. I’m not so frightened of people now. We all appear to have confidence, but inside we’re scared. I’m frightened of an invasion of privacy and perhaps afraid of being found out for not being what I appear to be. I’m pretending I’m Mary Wesley who writes these delightful books. But inside…’ She giggles. She adds that she’s afraid of being intimate.

Motherhood then. Did she, in turn, make a good mother? Or did she repeat her mother’s mistakes? She exhales deeply. ‘I fear I did make the same mistakes. I think life is repetitious. I don’t think I made a good mother. I should have been a much better one. My youngest son was 16 when his father died. I had no money at all, but perhaps I should have helped him more.’ She ‘adores’ her children, and once said she would die for them. Roger works for charities, Toby is a literary agent and Bill (begat by Eric) is trying to write film scripts. Would she have liked them to have achieved more? ‘I’m not ambitious for my children in that way,’ she says, a touch snappily. ‘I just want them to be happy.’

There is nothing, she says, to fear in old age. ‘It is stupid to put it off. It’s like death.’

She claims she can’t wait to find out about death, hoping it will be an interesting experience. ‘I believe there’s some form of existence after death for the spirit. I think we make our own hells when we die, we’ve created obstacles for ourselves.’

Does she think she’s led a good life? ‘Oh no,’ she says, hilariously. ‘I know I haven’t.’