Nor does he view his own disillusionment as an isolated case. "Many officers in undercover policing walk away with a very bad taste in their mouths. Even now, I can tell when someone's an off-duty cop just by listening to their conversation in a café. I just don't speak like that. All I had to do was put to use all the attributes I'd been unconsciously collecting all my life."

As someone so utilised, he was used against some major criminals, though most of his assignments seem to have been against wider criminal fraternities. So, for example, he worked undercover in the Moss Side in Manchester, building a picture of the criminal landscape there, and again later in Cambridge. What he never seems to have done is been deployed against a major criminal or crime family, such as the Adams crime family in London, or the Noonan crime family in Manchester. So, he never seems to have been deployed to infiltrate such an OCG and “bring them down”. This isn’t a criticism of Shay, or even the police. As a layperson, I don’t know. Perhaps that’s not how it’s done, or such an operation would be too risky. It was decided I needed a girlfriend so Nikki (a fellow UC officer) would be dressed to the nines in Jimmy Choo heels with a Gucci or Vuitton handbag. Notching up arrests didn't do anything for me. I had a turbulent upbringing, but I developed integrity and was always taught right from wrong, and to look after the weak and vulnerable. How were you able to keep your nerve with dangerous and unpredictable characters while you worked undercover in the Omega unit?

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Even going to the supermarket with my girlfriend was a f***ing drama," he said. "One time we were in my local Tesco when I spotted someone I knew turning into the aisle we were in. Based on intelligence he’d gathered, police arrested four gangland figures and found a fully loaded MAC-10-style machine gun and a couple of kilos of skunk in a car. But pretending to be a gangster played havoc with Doyle's home life. In 2009 the convictions of Colin Joyce and Lee Amos marked a turning point in the fight against gun violence in inner south Manchester READ MORE TRUE CRIME:• The violent rise and fall of gangster 'White Tony'... devoted Winnie Johnson's other lost boy

The soldier-turned-undercover policeman risked his life to infiltrate south Manchester's gangland. He was also on the frontline of some of the most high profile police operations in Greater Manchester history, including the Stepping Hill poisonings and the hunt for double cop killer Dale Cregan.Shay now works with organisations to break the stigmas around mental health, particularly in the emergency services.