She was never conventional. As a young woman, she led a gossip-column life, including affairs with Peter Sellers, Hywel Bennett and Dai Llewellyn, but it spiralled out of all control after her father’s death in 1990.

“I lost my reason,” she says. “He was my drug of choice, and when he died I lost my motivation. But I couldn’t cry for him, because I knew if I cried I would cry for everything else. I got terrible, terrible headaches, and then became addicted to painkillers.”

All that is behind her now, though she is still plagued by a bad back, which means she has to take to her bed a lot. She lives quietly in America for most of the year, renting an old Cape Cod shingled house near her mother in Martha’s Vineyard in Massachusetts. It’s only partly for financial reasons; “partly for mental health”.

The island is a place she knows and loves. “All the marbles roll to the edge,” she says. “The most interesting people live there. And I don’t mean the summer visitors from Boston. Occasionally, I go to their parties, in case I meet some newly divorced man, but it never happens.”

Even the feng-shui wall at home in Oxfordshire has failed her on that front. “All the pictures are arranged in pairs, and there’s a knight in armour. I was told it would mean I would meet somebody,” she says. “I used to have one pair of me and Sophie together, but she was horrified at the thought of ending up with me and put a picture of Brad Pitt between us. And now Maureen thinks the knight in shining armour might be Joan of Arc.”

She spends much of her time alone. Ned lives with Noble in Wheatley during term time; in school holidays and half terms he visits his mother. Sophie lives in Manhattan, Clover in LA, Luke is about to go to university in Britain. It’s an unorthodox arrangement that seems to suit them all, but it’s too expensive. She has high hopes that the new, “fantastic”, Charlie and the Chocolate Factory film starring Johnny Depp will boost the Dahl family’s finances, but in the meantime the house that she bought for about £500,000 is on the market for offers over £650,000.

She plans to buy another, new-build and cheaper house nearby for Ned and Noble, and eventually, when Ned has finished school in Oxford, a flat in Battersea.

She has learned a lot from her troubles. “I was in Asda just yesterday thinking how lucky I am,” she says. “I have this incredibly great package in life. I have attempted to screw it up so many times, but each time I’ve learned.

“My father dug me out of every ditch I ever dug myself into, and my bankruptcy would never have happened if he had been alive. The great thing about it though was I learned the value of money. I had to clean ghastly lavatories and pick up balls at the New Canaan Raquet Club (in Connecticut).”

Dahl may be wiser than she was, but she knows her limitations. There is no question of Noble ever leaving. “It would be like breaking up a marriage,” says Dahl. As well as looking after the family, Noble also cares for Dahl’s two dogs and three of the cats (the other four live in America with Dahl) and two parrots who are being sent to a parrot sanctuary because they are lonely. The only time that Dahl cries is when talking about their imminent departure. She’ll deal with it, she says, by staying in bed, asleep, until they have been taken away. Noble will be there to cope.

The Old Forge is for sale for £650,000 through Knight Frank, 01865 790 077, www.knightfrank.com

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