PAULA the evil Polish nanny has a lot to answer for. Her behaviour could have scarred Emma Thompson for life.

The double Oscar-winning writer and actress's memory of her childhood has been triggered as she promotes Nanny McPhee, the new film she's written and stars in.

This nanny arrives mysteriously as the answer to the prayers of widower Mr Brown (played by Colin Firth) for someone to tame his seven naughty children, a prospect from which even C4's Supernanny would recoil.

Naturally enough, Thompson is asked if she's ever suffered at the hands of a tough nanny. "I remember having a particularly evil au pair who ate all my Newberry Fruits when I came out of hospital after a tonsillectomy when I was seven," she recalls.

Later in the conversation, it emerges that Paula's brazen consumption of young Emma's sweets had repercussions. "After the incident, I ran away from home," she says. "I took my sister away in the middle of the night with a bag full of McVita stuck together with Marmite. We left the house, went round the corner, had a sandwich and went back. Paula came down to let us in, my parents were away. Nothing more was said about it." Otherwise, she says she was "quite good" as a child, apart from one incident. "I did draw all over the downstairs toilet and I was made to stay up and rub it all off the walls," she says.

Thompson has spent eight years trying to bring Nanny McPhee to the screen. The film is based on a series of English children's books, the Nurse Matilda series by Christianna Brand. Along the way, the novels underwent a name change.

"It took a long time to write," she explains. "I think screenplays should take a long time, personally. So many have not had a long time spent on them that should have done.

"As journalists, you know if you're writing something that's going in tomorrow about something that's happening today, it's a totally different matter to writing a novel or screenplay which you're trying to design to appeal to adults and children.

"It started off as an adaptation, but it's not - it's based on. It took a long time to get right. We kept thinking we were getting there. I did get five years into the process and was sobbing down the line to the producer, 'this is what it's going to be, I can't change it any more'."

"I don't know why it was such a struggle. I can only imagine that it was because it's difficult today to write for everybody and that was my loadstone. My dream was to write something everyone would enjoy."

She was only too aware that her late father, Eric Thompson, wrote one of the best loved children's TV programmes, The Magic Roundabout. They were five-minute episodes rather than a feature-length film but she was aiming for the same feeling.

"He never talked down to us when we were children," she says. "It's not always appropriate to talk to them as adults, but it's inappropriate to talk to them as if they don't come from the planet Earth.

"I've always wanted to write something in homage to what he wrote in The Magic Roundabout," she says.

She had no particular affection for the Nurse Matilda books growing up. "I'd like to say they're a seminal part of my childhood, but they're not," she says.

"However, I did read them when I was little and remembered when I picked up one of them when I was dusting. I sat down, read it and thought there was something in the book that would make a very good film. But I can't say they were completely beloved childhood memories."

She argues that many children's stories are concerned with a sense of loss. The Brown children in Nanny McPhee, for instance, have no mother. She also recalls liking children's films that had a slight element of romance, whether it was Mary Poppins and chimney sweep Bert or the father in Chitty Chitty Bang Bang and Truly Scrumptious.

"There's that sense of people building something and the repair of the family in children's stories. It's at the root of a lot of fairy stories as well. I bunged everything in the script - Pygmalion, Cinderella, there's everything, because those themes are endlessly fruitful."

Nanny McPhee is no oil painting with her prominent tooth, facial moles and generally unpleasant appearance. Yet Thompson wanted to play the role from the start. "Funnily enough, a kid came up the other day and said, 'did you mind being ugly?' and I said, 'no, I loved it'. It wasn't until I was asked the question that I realised how much I did love it. It was lovely being ugly, but at the same time you're the same person inside."

The look was achieved through prosthetics and make-up. It took just an hour to achieve, less than it took to transform her into Elinor in Sense And Sensibility.

Her own daughter, five-year-old Gaia, took her mother's new look in her stride. "She came on the set a lot and didn't notice any difference at all," reports Thompson.

"The first time we screened the film we were still in the process of editing and the ending was a bit different, perhaps sadder because we emphasised Nanny McPhee leaving, and Gaia was inconsolable. We play Nanny McPhee at home and she always says, 'this is the Nanny McPhee who never leaves'."

If this film is successful, a sequel could follow and Thompson is up for it. Because Nanny McPhee can move through time, the possibilities are endless. "The next one would be fantastic, I'd set in during World War Two and the kids would be evacuees. The mother would be at home and the father at war. There could be something powerful in that," she says.

l Nanny McPhee (U) opens in cinemas on Friday.