I meet Samantha in her home in north Wales, where she lives with her father, Gareth, 57, a maths teacher in a comprehensive school, and mother, Patricia, 44, who works in IT. Her sister, Alice, who is four years older, has recently graduated from university and lives nearby.

Now 17, Samantha is pretty with porcelain skin, a soft accent and a demure manner. The only hint of her former problems is an encyclopedic knowledge of make-up. She says her BDD has abated. “I want to look nice but it doesn"t really bother me that much any more. I"ve got friends again.”

The warnings started in autumn 2006, when Samantha started secondary school. She had changed schools two years earlier, and now felt on her own again because most of her friends were going to a different school. “I was really nervous and I was angry with Mum and Dad because I had taken so long to make friends and then I was leaving them again.”

Samantha began to experience strong feelings of anxiety. “I would drive her to school and I had to reach a bridge in a village by the time the quiz came on the local radio, and if I didn"t she would refuse to go to school,” Patricia says. Samantha became convinced that her mother would have a traffic accident. Getting to the correct spot was a way of safeguarding her mother. “I was very close to her and I didn"t want to be away from her,” Samantha explains. A GP diagnosed obsessive compulsive disorder (a cousin of BDD), and Samantha was referred to the local CAMHS where she had one-to-one sessions with a community psychiatric nurse. “I asked the doctor how long it would take to sort out,” Gareth recalls. “He said six to nine months. But it didn"t quite happen like that.”

Towards the end of Year 7, Samantha"s group of five friends split into two factions. Insults were fired back and forth. She was teased about her “chubby” cheeks - “I was one of the shortest and fattest” - and for being so quiet. The other girls shrugged the insults off. But Samantha started getting panic attacks every time she went to school. “I didn"t want people to see me. I worried about what people thought of me.”

By early March 2008 her OCD was under control and she was discharged from CAMHS. But then one morning she came downstairs with her shoulder-length hair chopped so violently it looked as if she had been assaulted. “She handed me this plastic bag and it had all her hair in it,” Patricia says. “She had cut her hair off overnight, big chunks of it.”

“I was trimming away bits to try and change the shape of my face,” Samantha says, “but then I got frustrated because it wasn"t even on both sides and I just kept going, cutting away.” The hair-cutting incident was followed by more strange behaviour. She would spend long hours looking in the mirror in her bedroom. If she wasn"t looking in her bedroom mirror it was the one in the hall, or wing mirrors on the car, or any reflective surface - a spoon, shop window, bottle of Coca-Cola.

In late March 2008 Samantha refused to go to school at all. “When a child says no, it"s the most powerful thing because we literally couldn"t do anything about it,” her mother says. Samantha"s formal education ended in term two of Year 8. She was given a diagnosis of BDD by a consultant child and adolescent psychiatrist from their local CAMHS in April 2008, and from March to May that year she mostly confined herself to her bedroom.

This was the room where she would study photographs of celebrities, ticking off their attributes on a mental checklist. Taylor Swift: “tall, thin, balanced body, no lumps and bumps”; Tulisa: “big boobs, thin body, blond hair, tanned”; Beyoncé: “quite big and curvy but everyone finds her attractive and perfect”. She would then feel wretched because she was “just plain and fat and disgusting”. This was the room where, when her GP decided that she should attend a child psychiatric unit in April, she pushed a chest of drawers against the door and screamed, “No!” The room where her various grooming rituals - trimming her fringe, plucking her eyebrows - were joined by cutting, as she started self-harming. This is the room where, in September 2008, her mother found empty packets of paracetamol and antihistamines.

“I remember being quite disappointed it hadn"t worked,” Samantha recalls. “I couldn"t even kill myself properly, and that made me feel worse.”

A community psychiatric nurse visited for an hour once a week to talk about her feelings and to try to take her out. She also had a home tutor, was put on Prozac, and was referred to a child and adolescent psychiatric clinic a 20-minute drive away, where she received group therapy with other teenagers suffering from such disorders as depression and anorexia. The plan was for her to attend daily, but she would often refuse to go. The family went to the unit, too, for family therapy once a fortnight. In October 2008 a second psychiatrist diagnosed BDD. In January 2009 she was given a morning place at a specialist education unit and an afternoon at the day unit. But attendance at both was erratic. “She was not coping well,” Patricia says.

Samantha then took another overdose in April 2009, and in May her parents found her dressing-gown cord hanging from a tree. (“I put it out a few days before,” Sammantha says, “to prepare.”) In June, after her third suicide attempt, she was admitted to a high-level psychiatric unit a 45-minute drive away, where she was an inpatient for seven weeks. She was treated with Sertraline, an antidepressant, but was given no specific treatment for BDD. Once discharged, she continued to be treated at the day clinic and by a community psychiatric nurse at home.

During this period it was difficult for the family to operate at even a basic level. The most routine events became fraught with difficulty. Samantha wouldn"t leave the house until she had completed her checking rituals. “We would plan to go out for a meal, and we would be waiting and waiting for Samantha to get ready, to the point where we couldn"t go,” Gareth says. Patricia removed all the mirrors in the house.

It was exhausting and rather than getting better with treatment, Samantha"s condition got worse. The first suicide attempt had put the family into a constant state of red alert. “One of us would always be here with her, we couldn"t leave her on her own,” Gareth says. Samantha defiantly searched for knives and pills. Gareth decided to buy a safe.

Patricia and Gareth are still stunned by what happened. They emphasise how out of place they felt in this “weird old land” of mental health services; how “they didn"t fit into the box” of a dysfunctional family. They see themselves as very close, very stable. They met when Patricia was 19 and Gareth 31, and are in many ways polar opposites: he is pragmatic, likes to make lists, is self-contained; she is volatile, creative, open-hearted. Patricia works hard at being a good mother, as her own parents separated when she was seven. “I know how important it is to provide a stable base for the children.” Despite this, they still went through three months of marriage counselling to help them cope with the stress.

In June 2009 Patricia gave up her job. “I couldn"t be relied on. They were asking me when she was going to get better. And I said, I don"t know.” But Alice was still at university and needed her rent paid, so Patricia worked from 10pm to 4am at a supermarket. “It was so I could be with Samantha during the day, but then I couldn"t sleep because I had to keep an eye on her.”