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Determined to help Mushy, the Programme, which has thousands of "graduates" worldwide, including hundreds in the UK, contacted Mushy via Channel 4. The result was Stammer School: Musharaf Finds His Voice, in which Mushy and four other young stammerers attend one of its four-day, privately funded, residential courses.

From the first meeting, when the group was filmed stuttering through answers to basic questions, it was obvious that Mushy was very far from cured.

"In more than 10 years leading courses, I've met two people not able to say a single word on the first night and Mushy was one of them; he couldn't even get out the first letter of his name," says Whincup. A former stammerer himself ("I couldn't even say 'good morning' "), Whincup went on to have his own local radio show and is now a university lecturer and motivational coach.

During the course, participants learn methods devised 20 years ago by US-born Dave McGuire. Because its techniques aren't yet supported by official research, the Programme isn't endorsed by speech therapists, although many still recommend it.

According to Whincup, stammering has both physiological and psychological causes. "The theory is a misfiring left side of the brain confuses messages to the voicebox and then, when you become aware of it, you're hit by the psychological impact and it gets worse."

Participants learn "costal" breathing used by opera singers, where air enters the lungs and the stomach expands. They also study a psychological technique known as "non-avoidance", where they confront their stammer, rather than dodging "difficult" words. On day three, they're sent out into the street to talk to 100 strangers. On day four, each speaks to a packed room.

Alongside Mushy was Vicky Croft, a 23-year-old recruitment manager from Liversedge. Until Christmas, Croft was an "outgoing person, you couldn't shut me up". But a small stroke left her stammering.

"It was shattering," she tells me, her speech now only faintly stumbling. "Often I'd have to text my mum when I was sitting right next to her, because I couldn't say anything. Once they wouldn't allow me into a bar, they thought I was too drunk. I thought: 'Am I going to be a recluse for the rest of my life?' "

Initially sceptical, Croft was addressing a crowded pub within days, leaving the production crew in tears. "I've never not cried during a course," says Whincup. "It's a very emotional thing, seeing people who'd lost hope, find this confidence."

Viewers' tears, however, will mainly be reserved for Mushy who, since the Programme, has been able to further his dream of becoming a teacher; returning to his old school, Thornhill Academy, for work experience.

His battle's still far from won, however. My meeting and phone call with him are cancelled – presumably as too stressful. "Stammer school has helped me a lot," he eventually emails. "It's giving me the chance not to overcome my stammer but to control it."

Relapses are common, but, like all McGuire "graduates", Mushy is allowed to return to as many Programmes as necessary. "Mushy made the progress you would hope and he has his whole life ahead of him," says Whincup, who's in constant phone contact with him. Mushy sums up: "If I could speak fluently, I would show people who I really am."

Stammer School begins on Channel 4 at 9pm on Thursday August 28

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