The mission: Can an underweight Maggie O'Farrell persuade diet clinics to take her money? Please step on the scales ...

Before I embark on a mission to expose diet clinics as a band of mercenary charlatans, I go and see my GP so as to be armed with official statistics. I stand shoeless against his chart. I'm 5ft 8in and, after some interminable fiddling with the dial on his scale, I am told I weigh eight and a half stone. "In other words," he says as he recaps his pen, "you're underweight by a stone at least."

"It's always the most difficult. That last half stone," a chirpy Antipodean is saying to me, his head cocked at a sympathetic I've-been-there-too angle. He is, his business card informs me, an official distributor of a diet nutrition plan. He is also one of those people that likes to use your name a lot. "And how much do you weigh now, Maggie?" I tell him. "Eight and a half stone, Maggie. So your target weight is eight. Am I right Maggie?" "Yes," I say.

The diet plan he "devises" for me consists of three sachets of powder a day that mix with water to "create a beautiful drink". I'm also allowed a couple of evil-looking tablets. "It's giving your body the nutrition it needs without the calories," he beams. "My wife lost seven pounds in a week." And all for pounds 45 a month.

I am getting up to leave when he announces, "Now we can watch the video," and switches on a TV in the corner. Overweight Americans leap up on a stage to emote and weep over how this diet plan has changed their lives. "I did it! Yeah! I did it!" screams a woman holding up a beautiful drink packet as if it were the Holy Grail. The man reaches for the remote control: "Perhaps this is a little too much for us Europeans," he says with his Mad Max twang. I am intrigued. "Are you a European?" I ask. He coughs, embarrassed. "Well, my wife is."

The second person I see is a woman with a barely audible voice who, or so she whispers, "works in conjunction with a clinical psychologist to work out personalised health plans". The first thing she asks me to do is strip down to my underwear, about which I'm dubious, but the words "psychologist" and "health" have given me hope, so I do. We both watch as the scale settles halfway between eight and nine stone. She notes this down. "And what's your target weight?" she murmurs without appearing to move her mouth. "Seven and a half," I say recklessly. She doesn't even flinch. "I'll need a seven-day record of your eating habits," she says, "then you'll have a two-hour appointment to discuss your daily plan at pounds 30 an hour."

At the third clinic, the rather maternal woman with pencilled-on eyebrows at the reception desk looks me up and down as if I have an infectious disease. "Ye-es?" she says, managing to give the word about three syllables. I explain I'd like to lose half a stone. "And how much do you weigh now?" she enquires. I tell her. "And how tall are you?" she snaps, "Five seven? Five eight?" I nod. "She won't give you any medication!" she shouts, scandalised, as if I've told her I fancy my chances with her husband, "She won't give you anything!" "Who?" I ask. "You're underweight! She'll wonder why you're here." "Who will?" I ask again, but she's on a roll now. "She'll tell you to eat more!" I start backing away. "Anyone would!" I consider telling her how untrue this is, but don't. "Thanks," I say, as I disappear through the door, "Bye"