Diets are usually about self-mortification. Here's one that's an exercise in self-gratification and, says Clare Garner, it helps to like chocolate.

Two large, sickly desserts arrived at our table: one brandy snap basket containing malted ice cream, and a sticky toffee pudding topped with a dollop of vanilla ice cream. This in itself was cause for alarm - the two women opposite claimed to have discovered the secret of slimming - but yet more bizarre was that they were, apparently, on their starters.

Professor Anne de Looy and Sally Ann Voak, weighing in at a collective 17st 2lb, make an unlikely alliance. The former is Britain's first professor of dietetics at Queen Margaret College, in Edinburgh, and the latter is slimming editor for The Sun. But together they have devised "System S", a diet programme in which "S" stands for science, not starvation, self- denial or sorry you can't eat that Mars bar. Equally, it could stand for "sugary foods".

It is founded on the principle that we eat too much fat and not enough carbohydrate and the motto of System S is "carbo first" (taken to its logical limits, this means pud before main). Their argument goes that since there are limits to how much bread, pasta and rice we can consume in the West, we must up our complex carbohydrate intake with a simple one: sucrose. Consequently, cravings for fatty foods subside.

"As we are surrounded by sucrose-containing foods it makes sense to use them to our advantage," says the introduction to System S: the scientific breakthrough for lasting weight loss. The ensuing two pages of trademarked "Carb Boosters" reads like a list of no-go areas.

Golden syrup, marshmallows, Arctic roll, liquorice allsorts, cola drinks or other sweet fizzy drinks, meringue nest, mini Mars bar, ice cream, After Eight Mints ... without these items the diet just won't work. And don't forget your statutory daily glass of wine or beer - or sugary non- alcoholic drink if you prefer.

Any diet book which maintains "You can eat out and enjoy a delicious dessert, serve up lavish, tempting meals when entertaining friends and join in when the rest of the family tuck into sweet puddings" is bound to elicit an extreme response of approval or disgust.

Personally, I liked Anne and Sally before I'd even met them. Any diet which says I can "live a normal life while slimming" has potential and any diet which says I must eat chocolate eclairs, I can live with.

"It's anti-intuitive and too good to be true," said Professor de Looy, tucking into her sticky toffee whatnot. "But we need our puddings." In these no fat/low fat days, such an affirmation goes against the grain, but I'm prepared to listen. Professor de Looy objects to modern usage of the word "diet". "Diet actually means the foods that you choose. Sally and I are not on slimming diets but we're choosing foods in the way of System S."

Professor de Looy and her team at the Centre for Nutrition and Food Research in the Department of Dietetics and Nutrition at Queen Margaret College, have given "System S" a gold star for nutrition. In 1996 they tried it out on 60 volunteers in a controlled 12-week study in the village of Fatfield, Tyne and Wear.

As a control group, 30 of the volunteers followed a traditional healthy, low-calorie, low-fat, high carbohydrate diet plan which was devised in 1991 and has proved successful both for those who wish to lose weight and for those who want to maintain their weight loss. Ms Voak's well-known "Fatfield Diet" is now recommended by doctors and nutritionists.

The remaining volunteers followed "System S" - a similar daily menu plan, but with the addition of two or three Carb Boosters every day. After a couple of weeks, the "System S" dieters had lost more weight than their fellow slimmers. (The average weight loss on System S is one pound a week.) Fifteen months on, they are still losing or maintaining their weight and "have learnt to love food and eating and no longer fear that they could at any time blow their chances of losing weight successfully by giving in to temptation".

People are reluctant to sign up to "System S". They fear that a licence to eat sugary foods will mean their consumption spirals out of control. In some cases, indeed, it did. "If I have one chocolate biscuit, I'll eat the whole pack," said one volunteer, explaining why the diet didn't work for her. However, with the help of "attitude exercises", such as nibbling a Carb Booster in bed while visualising yourself looking slim and shapely at a party, most volunteers overcame the initial reluctance without swinging to the other extreme.

It worked in the case of the chocolate factory workers who volunteered as guinea pigs. "They are given free sweets and chocolates every day. You can imagine the guilt and problems," said Ms Voak. "Once they went on `System S' they not only lost weight but they adopted it over Christmas, which is remarkable to me."

Ms Voak and Professor de Looy insist their diet is not "faddish". It is, they say, a "lifestyle" - and not a bad one at that, I thought, as they ordered a bottle of Pinot Blanc D'Alsace. As for their second course, smoked salmon was out because it didn't contain carbohydrate, as was the mussels dish and the tomato, goat's cheese and wild rocket salad, for the same reason. The only dish which qualified was the leek and potato soup.

While Professor de Looy was big on the scientific validity of "System S", Ms Voak stressed the sensual. "The important thing is that you really enjoy the sensuality of food," she said, adding that she had enjoyed the "naughty phallic shape" of her pudding. "It's the next best think to sex - preferably two at the same time." The prof perked up: "Oh Sally, what did you have in mind?" Another experiment, perhaps.

"System S" is based on that well-known law of desire. We all know that as soon as a food is labelled as forbidden it becomes infinitely more enticing and inviting. The problem lies in our perception of sugary foods as "treats". Ms Voak has a New Year's resolution never to write another diet with "treats" in it.

But is that not precisely what she has just done with her concept of Carb Boosters? "You've got to consciously work the right way before subconsciously," she replied, reaching for the right metaphor. "It's like when you practise breathing classes before having a baby. When it comes to the right time you will be able to do it naturally."

Susie Orbach, author of Fat is a Feminist Issue, applauded the diet's attempt to normalise our relationship with food. "Of course any approach to food that doesn't categorise certain foods as good and bad per se I would welcome, but the crucial issue is what interferes with our own methods of satisfaction and hunger and what are the psychological reasons why we turn to food.

"Unless we've questioned those we'll always be looking for the next scheme to avoid food or manage it, rather than living in a reasonable relationship with it. The real challenge is to figure out what we want when we go to bed. Is it After Eights or is it nothing?

I know which I want.

"Clare's is much more fun," commented a food-combining colleague, as I counted out my Jelly Tots (essential Carb Booster, you understand) at my desk. Indeed it is. But, I ask myself, does it work?