Sarah, duchess of York: Red alert

You got to hand it to Fergie: she's a game old girl. Next week the world will be exposed to the tale of how Sarah, Duchess of York, left her £2.5m New York apartment overlooking Central Park to travel to Hull to lecture the underclass about healthy eating.

She landed in the Preston Road Estate, which has such high levels of crime that it's known locally as Beirut. She spent six months, on and off, living there with the Sargersons, a family of six, who survive in a council house on benefits of £80 a week. The opening dialogue of the fly-on-the-wall ITV show The Duchess in Hull conjures the picture:

The Sargersons wonder who their famous visitor might be.

Mick: "We've got Fern Britton, Nigella Lawson, Ricki Lake, Vanessa Feltz and Kerry Ke Tanawa, whatever her name is..."

A red-headed woman arrives to blank looks.

Duchess: "Did you think it was Oprah Winfrey? Oh my God. That's so sad. Do none of you know who it is?"

Tonia: "I've seen you but I can't remember your name."

Duchess: "I married Prince Andrew, right. Diana was my sister-in-law. You've heard of the Queen of England have you? The Queen was my mother-in-law."

Mick: "How do you do. Nice to meet you."

Mikey, 14: "She's all right but I reckon it's just a publicity stunt."

Who are we to contradict? Viewers of prime-time ITV on Monday and Tuesday can look forward to learning how the Duchess stopped the Sargersons kids putting tomato ketchup on their mashed potato and sent them all down the gym.

The only thing that seems to have fazed her, causing her to walk out of the press preview after 10 minutes, was that the opening shots showed her exhausted and without make-up after a gym session. "People need to see the 'real you'," smooth-talked the producers. The Duchess was mollified.

But no mockery, please. Here, at last, is something that the woman who once went by the title Her Royal Highness, the Princess Andrew, Duchess of York, Countess of Inverness and Baroness Killyleagh is actually qualified to do. For three decades now, Sarah Ferguson has struggled with eating disorders and her own body shape in an unhappy crucible of low self-esteem, family conflict, financial crisis, divorce and bereavement. Indeed, to many her most endearing characteristic has been her willingness to own up, in a very public way, to her battles with these very private demons.

"I would love to do more work in this country," she said this week in an interview with Radio Times, "to just share some of the knowledge that I have gleaned over the last 15 years – how I'm still standing really."

Not that there wasn't something endearing about the old Fergie. The fat, frumpy, sporty, bread-roll-throwing, wildly guffawing, mildly embarrassingly It's a Knockout bumptiousness. Even the Queen had a bit of a soft spot for her in the early days, realising that what had attracted No 2 Son to his spouse was in part, what one royal insider called "her more novel qualities – such as standing on chairs and telling jokes". She was, in contrast to beautiful broody Diana, feted inside the palace as a "breath of fresh air".

There was something hearty and bouncing, shortly after her marriage to Prince Andrew in 1986, about the enthusiasm with which she learnt to fly both planes and helicopters so that she could "discuss work with" her Navy chopper pilot-husband. When she became pregnant in 1988 with Princess Beatrice (now fifth in line to the throne), she refused to give up skiing. And when an IRA enthusiast lunged at her with an Irish tricolour on a visit to New York that same year, she shrugged him off and continued on to the gala dance.

But there is only so much fresh air that a palace needs blowing through it. Though Sarah was a descendant of Charles II, on the bastard side of the blanket, she was a commoner in more senses of the word than one. Three years on the young bride, with her yo-yo weight and peculiar eye for fashion, had become branded as the duchess of tack and tactlessness. "Vulgar, vulgar, vulgar," said the Queen's former private secretary, Lord Charteris.

The thing about Fergie is that she is a fighter. That much was clear this week when she publicly laid into the Daily Mail columnist Allison Pearson who had made rude remarks about Beatrice's curvaceous figure. The woman who with heavy self-deprecation refers to herself as "fat frumpy Fergie" revealed that she had tried to make contact with Ms Pearson, but that the journalist "had not had the good manners to return my calls". There followed some insinuations about the size of the journalist's bum. Why couldn't she have written, Fergie fulminated, about how the princess had struggled with dyslexia from the age of seven and still managed to get an A* in history and a place at Goldsmiths University. You can see why the Duchess was named Mother of the Year recently by the American Cancer Society.

But then motherhood is a touchy subject for Sarah. Her own mother had told her when she was a pudgy and freckled girl that she was ugly and warned her not to look in the mirror. When Sarah was 12, her mother ran off with a polo player from Buenos Aires. Years later, the Duchess revealed that she had been psychologically damaged by her mother's behaviour.

"I have a weight problem," she told one audience in the role she has championed for the past 11 years with WeightWatchers International. "It started when I was 12 years old. I am not frightened at all to say that weight is the bane of my life. I know the mood swings, the depressions, how it feels to look in the mirror and pinch all those inches and what it feels like to be too embarrassed to get out of bed in the mornings." Her mother's elopement left her feeling abandoned and yet guilty for resenting her mother's happiness. She became a compulsive eater and crash-dieter. Even on her wedding day, she disclosed, she had a diet-induced migraine.

So it continued through her marriage. Prince Andrew was away at sea all but 40 days of the year. She ate for comfort. "Fifteen stone later, the press in Britain started calling me the Duchess of Pork and said 82 per cent of men would rather sleep with a goat than Fergie. It was horrendous and the more I read, the more I believed I was a disaster. To everyone else I was fun, jolly, great but all those who have a weight problem can hide their feelings brilliantly.

"Then," she told the crowd, coming to her evangelical peroration, "I joined WeightWatchers." She was saved. Alleluia.

Self-mockery has always been part of the Fergie armoury. This is the woman who in her single days described herself as a "night owl who enjoys a hoot". At one point she described her fat bottom as looking "like live ferrets jumping around in a bag". It was that kind of boldness that helped her to shrug off the notorious toe-sucking incident in which she was photographed by Daily Mirror hidden cameras topless with her "financial adviser" John Bryan. Coming after reports of encounters with a number of men while her husband was away on military or royal duties, it turned a separation into a full divorce. She had been a royal princess for just 10 years.

She was left with huge debts. Her divorce settlement was reported to be just £300,000, plus a house. Yet she had run up an overdraft with the royal bank Coutts of £4.2m. "She has overspent," said her father blithely. "Most daughters overspend." But Sarah the survivor started to earn a living to pay off what she owed.

First came the books, Budgie the Little Helicopter, an autobiography, a series including Dieting with the Duchess for WeightWatchers and the revealingly titled What I Know Now: Simple Lessons Learned the Hard Way. She became an easy target for disdain because she turned into an even greater self-publicist, though she had to be because whenever her daughters went to stay with their father and grandmother at Balmoral or Sandringham estate, the Duchess was reportedly sent their laundry bill.

The books were followed by product endorsements and, horror of royal horrors, employment. Most of it was based in America where she has lived for the past 12 years. She lent her name to Wedgwood china, her own line of Moissanite jewellery and an American fragrance firm called Bath & Body Works. She went on the lucrative US public-speaking circuit. She became a special correspondent to the NBC Today Show and stood in for Larry King on his chat show. And if the Royal Family sniffed that her commercial activity was cheapening the monarchy, she had at least acquired enough discretion in her old age to restrict most of it to the US.

She felt at home there. "I love the fact that [Americans] embrace me," she told Larry King in 2003. "I said I was sorry for whatever I had done in the UK, and they embraced me and said, OK, we'll give you a second chance. They have given me my life back, the American people."

But the visits to Hull may well augur a more general return to Britain. "Americans embraced me and gave me confidence when I needed it," she told Radio Times. "But I want to come home. I like to think I can be part of the country again." She fancies herself as a health campaigner, she says. "I don't know if it will work. I didn't return before because I don't need to put my head in the firing line again."

There are some who will feel that is exactly what she is doing with her new TV programmes. "I could live in a council house and below the benefit line, of course. Anyone could," she told Radio Times expansively. "But I wouldn't smoke and I'd make it nice inside and eat green vegetables because I was brought up on a farm."

Maybe she hasn't changed that much after all.

A Life in Brief

Born Sarah Margaret Ferguson, 15 October 1959.

Early life From an aristocratic family, descended from King Charles II. Married Prince Andrew, Duke of York, in Westminster Abbey in 1986. Received a private pilot's licence a year later, making her the first woman in the Royal Family to do so. Gave birth to Beatrice in 1988 and Eugenie two years later.

Career Couple decided to separate at the start of 1992. Compromising photographs of her with US businessman John Bryan then appear in the Daily Mirror. Divorced Andrew in 1996. Faced hostile press coverage, and moved to the US to start a career. Projects included her own lines in jewellery and perfume, and acting as a spokeswoman for WeightWatchers. Also set up a series of charitable foundations, mostly for vulnerable children.

She says "Every single minute of my day I think that I'm fat, ugly and disgusting ... that I'm unworthy, that nobody likes me."

They Say "She's a real person with real issues. We wanted to show her journey." Nick Bullen, executive producer of The Duchess in Hull