Apart from the frowning statue of Sigmund Freud in the car park, the uninitiated would not guess what happens inside the Tavistock and Portman NHS Trust. It looks like an anonymous, unfriendly office block. Yet last year this world-famous clinic in north London treated more than 4,000 children suffering from mental health problems. The issues they arrive with make devastating reading: depression, anxiety, self-harm, mood swings, violent outbursts, suicidal thoughts. At a time when the number of teenagers taking their own lives is at a 17-year high, the Tavistock is at the forefront of exploring young minds.Take, for instance, Josh, one of an estimated half a million British children said to be at risk from early trauma. One of this 11-year-old’s earliest memories is of scavenging in the kitchen looking for anything that he and his two younger brothers could eat. The boys were starving. Their parents were either partying somewhere else in the house, or sleeping off their hangovers. With no one to look after them, Josh — barely four at the time — took charge. In the back of a cupboard he found what looked like a can of soup. It was a devil to open but eventually he managed to prise off the lid. Food at last.

It was gloss paint.

Josh is one of the lucky ones. Seven years on, having been put into care and placed with four sets of foster parents, he and his brothers are now happily settled with their — frankly amazing — adoptive parents, Stig and Phil. They live in a smart, spacious house in London that echoes to the sound of rowdy footsteps on the stairs and cheerful demands for attention. Yet there is no getting around the emotional fallout of years of neglect; and Josh, the eldest, is the most scarred.

After a brief honeymoon period, Josh was frequently violent in his new home. Stig recalls: “He would try to headbutt me, bite me, kick me. His eyes would pop out of his head, glaze over. I’d restrain him, sometimes for hours. Then he learnt to wet himself deliberately, so I’d be forced to let go of him.” His cortisol raging, Josh barely slept. At night Stig and Phil would lock their own bedroom door for fear of what he might do next. They recall a beach holiday when he held his brother Timmy’s head under the sea and refused to let go; episodes when he would torture the dog.

His local primary school was at a loss. This bright, round-faced boy would walk out of class, throw chairs and tables, and disrupt lessons. His crucial formative years as a baby when the brain is developing had been frightening and unpredictable. Josh was wired for either fight or flight. Unconsciously he needed to draw everyone around him into his own chaos — because that was all he knew.

This is where the Tavistock comes in. Three years ago Stig and Phil discovered Gloucester House, a unique school run by the Tavistock and funded by the local authority and the NHS that specialises in 4 to 14-year-old children who, like Josh, have been excluded from mainstream education. Some of them, like Josh, have come from the care system. Housed in a ramshackle building, with security cameras at the entrance, it currently has 18 pupils, 17 of them boys.

It is here that Josh received up to three sessions of psychotherapy a week, that there was an air-conditioned padded pod where he could go when his emotions tipped over. Here he could thrash about without getting hurt — or hurting anyone else.

Visit Gloucester House and it swiftly becomes clear that it’s unlike any school you’ve been to. The children sometimes swear and shout and run about. For an outsider, there are moments when it feels as if you’re on a mutinous pirate ship. Furious altercations bubble up fast like milk in a pan — but then they pass and the children are encouraged to talk about what happened. The staff — teachers, clinical nurses, therapists — are firm but they never raise their voices.

When the head teacher, Nell Nicholson, is asked, “Isn’t this a school for naughty children?” she beams. “I don’t believe in naughty children,” she replies.

For the first time, the Tavistock has allowed cameras inside both to reveal how it works and to shine a light on a generation of children and teenagers who are experiencing a shocking rise in mental health problems. Traditionally, the counselling room is private, sacrosanct. It has taken more than three years for the production team from Channel 4 to gain the trust of both the medical profession and their patients, some of them as young as nine. It makes for harrowing, tense viewing, but at a time when the government spends just 0.6 per cent of the NHS budget on children’s mental wellbeing, and child and adolescent mental health services (CAMHS) have had their budgets cut by £85 million since 2010, the series couldn’t be more urgent.

“We are seeing more and more young people in distress, taking risks, self-harming. Things could get worse. We need to act now,” says the Tavistock psychiatrist Dr Alex Sales. “The current so-called mental health epidemic is a reality.”

Josh would bite me, headbutt me, kick me. His eyes would glaze over

In the past year Childline has received almost 20,000 calls from children plagued with suicidal thoughts. Six times more girls than boys have rung the helpline, a ratio echoed by the Tavistock, which currently sees more girls than boys. The Times’s campaign highlighting the problem of children’s poor mental health, Time to Mind, has called for more resources to be allocated to young people. Thousands of children are turned away from NHS mental health services every year because of cuts. Often accident and emergency departments are the only places left to go. A recent review by the children’s commissioner said that the system was “playing Russian roulette” with young people’s health.

When 15-year-old Maisie, featured in the Tavistock series, isn’t in a locked mental health unit some 100 miles away from home, she lives in Hessle, East Yorkshire, with her mum, Sally. She suffers from depression and anxiety, and self-harms. These problems have been exacerbated by the closure of the local children’s mental health unit in nearby Hull through lack of funding.

When she had her first breakdown, the nearest available bed was in a secure unit for 16-year-olds on a ward more than two hours’ drive away. Maisie was only 12 at the time. Sally recalls seeing her daughter standing on the windowsill shouting through the small open window: “Mummy, please don’t leave me, don’t leave me.” It was six days before Christmas.

Maisie’s mental health issues had begun after the death of her father. Friendships at school fell away. Her teachers discovered a notebook in which she was writing about her suicidal thoughts. One night, events suddenly escalated. She combed the house for something sharp to harm herself with: scissors, razor blades. She headbutted the floor. She was out of control. “It was like we were watching something on television,” her mother recalls. “It was surreal.” The way her mother describes the unravelling events, it sounds as though everyone — the police, the ambulance service, the person from the local CAMHS team, the staff at the A&E department — was out of their depth.

Since then, Maisie’s teenage years have been punctuated by being sectioned. There is an impression that she has been to every secure unit in the north of England and none of them has any answers; that Maisie is in danger of becoming frighteningly used to pinging between home and hospital.

Ask Maisie about her mental health episodes and she replies: “I just go into this trance of nothingness. There is a hole inside me that is quivering and shaking, and there is nothing I can do about it. It is just the way I am.” Meanwhile her weight has increased from the medication. When Maisie is at home, her mother employs someone to sit with her during the night because she doesn’t sleep. Some nights she takes her to the 24-hour Asda, just for something to do.

At home, pills, aerosols and anything sharp are hidden away. Maisie’s way of “coping” with her emotional pain is to sublimate her distress with the physical pain of self-harm. Her mother will go into her bedroom in the morning and dread the towels and sheets soaked in blood. In the documentary we see mother and daughter wrestle over some safety pins. When Maisie is swept up in her emotions, she is violent. Her mother is covered in bruises. Afterwards, she is full of tearful remorse.

This is parenting in a war zone. Sally has had to give up work as a primary school teacher. As a result of the loss of income, they have moved into local authority housing. She has fought to get a diagnosis but the experts can’t agree on what is wrong. One doctor has said that Maisie is on the autistic spectrum, another that she is suffering from some kind of post-traumatic stress disorder. What Sally wants is a definitive answer.

She is putting all her hope in the Tavistock, where she has campaigned for a year to get treatment. It takes a day to drive to London and back but she has begun taking Maisie for weekly meetings. Her psychotherapist, Sarah Helps, hopes that once Maisie’s needs have been assessed she’ll be able to set up a team locally to help her. Her concern is that “Maisie didn’t get the access to the right help at the right time.” Perhaps if she had, things would be much better.

Back in north London, Josh no longer goes to Gloucester House. He can’t dodge the aftermath of the neglect of his early years, but he has come a long way. Crucially he has learnt how to think before he acts out, and how to feel sadness rather than anger. He comes across as remarkably emotionally aware for his age. After three years, everyone batting on his team — the teachers, the therapists, his case worker and his parents — have decided that he is ready to move on.

Two months after we meet, Josh started mainstream secondary school. I texted Stig to see how his first day had gone. Brilliant, came the reply: “We’re so proud of him.”