Jeanne Mithieux is not half the woman she used to be. It’s hard to believe looking at her now, but 18 months ago she tipped the scales at 21 stone. Now Jeanne, 28, weighs just nine stone. It has been a life-changing transformation that has taken her from a size 32 to a size 12, from struggling up the stairs to her first-floor flat to training for the London marathon.

But Australian-born Jeanne, a teacher at Brockwell primary school in the London borough of Lambeth, is astonishingly matter of fact about it. “I always knew I was going to lose the weight. I’d been big for a long time - since before I was a teenager. But I always knew I would be a normal size one day. Somebody said to me the other day, ‘It’s a fairytale’. But because I’ve lived through it, it doesn’t seem incredible.”

The self-belief she now exudes wasn’t always so apparent. “The weight was always something to hide behind. But I had to get a certain level of confidence in other areas before I had enough confidence to lose it.” She talks about “the weight” as if it was something she wascarrying around and lost, like some excessbaggage.

“It’s like I had to be successful at school, and I was. Buying my flat I saw as a success, as was getting away from Australia and surviving and developing friendships over here.

“As a teacher you have to pretend you are confident all the time. I would come home exhausted.”

Millions of people every year try - and fail - to lose weight. The tempting promises of “a new you” offered by diet books (700 titles are published every year) keep us coming back for more. An estimated one in 10 of us is on a diet at any one time, but a much smaller proportion reach our target weight and maintain it.

Jeanne, who had never seriously attempted to lost weight before, chose the Rosemary Conley diet because it was “big on exercise” and had a fairly liberal eating regime. “You had to watch the fat but you could eat lots of fruit and veg - which I love.” She set a date when she would start the diet and made her target weight in stones - 9.5 - her personal ID number on the school photocopying machine - a daily reminder of her task.

“I told people for ages beforehand that I was going to lose weight because I was reaching the level where I knew I could do it. There was no one thing that happened and made me do it. Some people have a trigger - they may see a photo and think ‘that’s it, I’m losing weight’. That was never me.”

She talks about her old habits with a mixture of incredulity and amusement. “I used to eat rubbish. Caramel shortbread was my weakness. Now the thought of eating a huge amount of it fills me with horror. I loved crisp butties. Wotsits on chunky white bread with lots of butter. Mmmm, that was a real treat - and so easy when you get in from a day at school. That was followed by shortbread.”

Cutting out her calorie-packed snacks had an immediate effect. “You lose a lot of weight quickly when you start. But I was getting into the housework much more, like vacuuming. I would dance in the kitchen to the radio. I had much more energy. I bought a stairwalker machine but I weighed too much to use it. So I started with exercise tapes and then started walking. I used to do long walks - 18 miles. But they were taking up all of my time. So I started running.”

At first she was so embarrassed to go out that she would run round and round her living room. “The people downstairs must have wondered what was going on,” she laughs. Then she started jogging in nearby CrystalPalace park. Now she runs to school every day, runs 18 miles at weekends and will take part in the London marathon in a few weeks’ time (she has pledged to raise pound;2,500 for Parkinson’s disease). She says the idea still hasn’t sunk in but, having come so far in so short a time, 26 miles must seem like a stroll.

The pupils at her school will be cheering her on. They have been her biggest supporters and - in the way only six-year-olds can be - straight-talking critics. “Kids are honest. They would start asking me if I was going to have a baby. They thought you could only be that big if you were going to have a baby. And they would say ‘you’re fat’. They were never nasty, they were just factual. But the facts were enough to make me upset.

“Since I got back this year I have been classified as ‘not fat’. But my last class, my Year 2s, really saw me through it. They have been so lovely. When we had a a birthday party, they would say, ‘You can’t let her have that. You know she’s trying to get thin’.”

Certain incidents have become telling reminders of her progress. “One little boy came up to me in the playground and hugged me and he said, ‘Jeanne, I think you need to lose some weight’. I’d been trying for a couple of months by then and he said, ‘My hands can’t reach when I hug you’. About three months after that he was giving me a hug and he said, ‘Jeanne my hands can reach around you now’.

“Another kid went to the doctor’s surgery, saw a healthy eating brochure and brought it in for me. He’d gone through and highlighted all the bits he thought I would be interested in. He’d obviously spent a long time doing it. It was brilliant.

“Losing the weight has made me more energetic in the classroom. In PE the other day I was demonstrating on the balance beam. There was no way I would have got on the balance beam before because I am sure I would have broken it.”

Her diet became something of a talking point at school. “The parents got into it then. When the kids had parties they would come in with low-fat versions of the treats and say, ‘My Mum made this for you’. That was so sweet.

“You are on public view all the time as a teacher - so many people are seeing you all the time. But they were so encouraging. Some parents would come up to me and say, ‘Congratulations, we’re so pleased for you’. Others don’t recognise me.”

When she went back to Australia for the millennium celebrations, her mother came to meet her at the airport - and looked straight through her. “She had no idea. I was walking towards her and my mother was reading a newspaper. She looked up, looked at me and looked back down at her newspaper. It took her a couple of days to get over it. She kept on looking at me like I was a stranger. It was weird. The last time I saw my brother he was 14. Now he’s 18. He looked straight at me, caught my eye then looked away.”

Her new appearance surprises herself sometimes. “When I look in the mirror I know what to expect. But when I’m walking through a department store and I catch sight of myself in a mirror it’s a shock. Sometimes I’ll look at my stomach in the mirror and think ‘Oooh, is that me?’” In January she was named Rosemary Conley Slimmer of the Year. Losing the weight has brought her pound;2,000 in prizes, and media attention - a dizzying round of television and newspaper interviews. But most of all, she says it has made her happy.

“I thought I was happy before. But now that I am so much healthier I am so much happier. I feel like I’ve got so much more to live for - as if I’ve got a future. I’m going to do everything. Physically, I feel like you could put anything in front of me and I could do it. That gives me a great deal of happiness.”