Helena's secret pain

I meet Helena Bonham Carter in her local, The Hill, on Haverstock Hill. She is wearing a red tartan suit - a tight bolero jacket and long skirt - with lime pop socks and clumpy brown boots. Her hair is a bird's nest, her face free of make-up, with dark smudges under each huge, brown eye.

She carries a bag I thought at first was Luella Bartley but turns out to be something she picked up for five quid in Turkey. She is deep in conversation with an elegant, smartly dressed blonde woman, whom I later find out is Jacqui Jackson, the woman made famous in a documentary about how she has brought up seven children, four of whom have varying degrees of autism. You would think that Jacqui was the movie star, Helena Bonham Carter the harassed mum who has survived the past 20 years on three hours sleep a night, max.

They speak often on the phone - "It's like listening to a radio play when you get Jacqui on the phone, all these voices in the background" - and Helena occasionally lends Jacqui a gown for her more grand publicity outings. The reason the two have become close friends is that Helena has just played her in a new BBC drama, The Magnificent Seven, to be screened next month.

My immediate thought was that Helena's frail beauty and upper-class credentials were all wrong to play a single mum living in a tiny semi in Blackpool, but her performance is incredibly moving.

"Jacqui is not remotely a stereotype," Helena says. "She has a firstclass degree in social policy, is studying for a PhD, is a black belt in tae kwon do, she isn't fazed at all meeting me or coming to London, and all her kids have achieved so much more than was expected of them."

The eldest, Matthew, now 21, got the equivalent of three A-levels and is now in the Sea Cadets. Luke, 16, who has Asperger syndrome, is a regular conference speaker on the subject and has even written a book, Freaks, Geeks and Asperger Syndrome; Joe, 12, is a computer whizz and Ben, eight, who was never expected to walk or talk, is allergic to noise, light and clothes, and spends his life in headphones, goggles and a nappy, is coming on in leaps and bounds. The three girls, all frighteningly normal, are excelling in the arts and languages.

To research the role, Helena spent time not only with Jacqui in Blackpool ("her house is immaculate, not like mine") but with other mothers and their autistic children. "I had never had any experience of autism before and I would come home and look at my son, Billy, who is now two, and be absolutely paranoid, particularly because he loves Thomas the Tank Engine, and lots of autys love Thomas. But he is not very good at pointing, and autistic children absolutely love pointing."

Although Helena made her name at the age of 17 when she left Westminster School to play Lady Jane Grey for Trevor Nunn, and went on to star in films such as A Room with a View and Howard's End, the Northern accent wasn't such a huge leap. "Everyone thinks I have this aristocratic background because of my great-grandfather [the Liberal prime minister Herbert Asquith], but he wasn't posh, he was a Northerner!" she laughs.

So much about Helena Bonham Carter is not what it seems. She looks as weak as a sparrow, but in fact had to be inhumanly strong to lug around the various child actors on set (she says she never exercises apart from pushing a pram on Parliament Hill, finds triyoga on Primrose Hill " intimidating", but is going to join the gym up the road "in January, definitely").

There is a core of inner strength about her, too - she says she is incredibly "earthed" - because growing up from the age of 13 with her father in a wheelchair meant she didn't have time to rebel, or be a wimp. She just had to get on with things. Cope.

She wears a photograph of her very handsome blond son (Johnny Depp is his godfather) in a tiny silver frame around her neck, and strokes it constantly. Did making the film about Jacqui make her want to have more children? "I don't think I'll have seven," she snorts. "But I would like another one if I get lucky. I'm 39 now, so the odds aren't that great."

She met her partner, the director Tim Burton, when he cast her as a chimp in his remake of The Planet of the Apes in 2001; she has starred in four of his films, the most recent being The Corpse Bride and Charlie and the Chocolate Factory, although she adds wryly that he still makes her audition. She thinks Burton might have a tendency towards the autistic. "I bet lots of animators are Asperger's," she says, frowning hard. "Tim will kill me, but while making this drama I realised he has a bit of Asperger's in him, you start recognising the signs. We were watching a documentary about autism and he said that was how he felt as a child.

"Jacqui's Luke said that without any Asperger's the world wouldn't go around: they have application and dedication. You can say something to Tim when he's working and he doesn't hear you. But that quality also makes him a fantastic father, he has an amazing sense of humour and imagination, he sees things other people don't see. Billy is enchanted by him.

"We are both very untidy and chaotic as parents; I think Billy will grow up to be very strait-laced and proper, actually. I look at Billy and I don't think it's wishful thinking, but he will wrinkle his nose just like my father did, and I think, 'Oh God, I'm pushing my father round in a pram!'"

Helena's father, Raymond, died last year at the age of 74. In 1979, when Helena was 13, he had an operation to remove a brain tumour but suffered a stroke, which left him paralysed.

Before his illness he had been a merchant banker and a great lover of books but, over the years, he gradually lost his sight as well. His mind, though, remained sharp, right to the end. "It was hard, incredibly hard, but with his disability there were lots of gifts, in a way," she says.