She was equipped with a five-grand Louis Vuitton handbag and the air of a WAG wannabe," he says. They even rented a flat together in the gangland's heart and used it as their work base. "In reality, she was a brilliant operator and highly intelligent." Doyle would spend weeks at a time in the role and would only shed his persona for a few stolen nights a month when he returned to his real girlfriend who was living in their real flat, just nine miles away. "My work put untold stress on our relationship as I became more impossible to live with," he admits. Shay now works with organisations to break the stigmas around mental health, particularly in the emergency services. 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. The rise and fall of Dominic Noonan, the paedophile gangster who once ruled Manchester's underworld

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. He added: “We’ll never know who those bullets were meant for, but this crew didn’t carry guns for show. It wouldn't be long before Shay's prodigious talent caught the attention of the top. Then came the call that changed his life: an offer to join the secret Level 1 undercover unit known as Omega. And it was easy to see why they wanted him; he wouldn't have to stray too far from what he already knew. He had all the attributes of a professional criminal - the athletic physique of a cage fighter, the talk, the walk. Streetwise and fearless, he'd be a match for the most hardened villain. He was given a new identity, his DNA and fingerprints were removed from the national database, and so began the life of Mikey O'Brien. The biggest case of his career began just after midnight on May 25, 2012 when Mark Short was shot dead as he played pool in the Cotton Tree pub in Droylsden. All the years he had spent learning the ways of the street: how to talk, what to say, how to conduct himself in the company of criminals proved to be gold dust. "Most cops speak in cop language, even when they are off duty - any decent criminal can spot them a mile off," he says. There he unleashed a hail of bullets on the two unarmed officers, tossed a hand grenade on their dying bodies and then calmly handed himself in at Hyde police station. Pc Fiona Bone (left) and Pc Nicola Hughes (Image: PA)In the business world, you put experts in their field in charge of certain areas of operation. The police quite often don't follow that model," he says. "Ego hinders management in the police world." I welcome former British soldier and undercover police officer Shay Doyle to the show in this interview episode.

My home life suffered though. I knew when I started the job it was madness, and that’s exactly what it turned out to be – absolute f***ing madness.” Before joining Greater Manchester Police he had served seven years in the Army, and knew how to handle firearms and explosives, and execute a plan with military precision.Shay grew up on a tough Manchester council estate where drugs and gangs were rife. A life of crime would have been an easy path to take. So it went against everything that was expected of him when he joined the police. Some of the practices adopted by those infiltrating the world of activism I find absolutely disgraceful," he says. "It's a different world to that of organised crime." To add credibility to his cover story, he worked alongside Nikki, a fellow Level 1 covert operator who dressed up in Gucci and posed as his girlfriend. 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. It wouldn't be long before Shay's prodigious talent caught the attention of the top. Then came the call that changed his life: an offer to join the secret level one undercover unit known as Omega. And it was easy to see why they wanted him; he wouldn't have to stray too far from what he already knew. He had all the attributes of a professional criminal - the athletic physique of a cage fighter, the talk, the walk. Streetwise and fearless, he'd be a match for the most hardened villain. He was given a new identity; his DNA and fingerprints were removed from the national database and so began the life of Mikey O'Brien.