Rogue Bullet - Kirk Paterson Adam

Mason was the Army's best-kept secret. A reservist with a gift for long-range precision and a life he finally thought was safe. But when an IRA splinter cell executes his fiancée, the "quiet life" ends. The soldier is awake, and he's seeing red.
When the security services approach him with an offer of training and a clean shot at his fiancée's murderer, Mason doesn't hesitate. He goes to ground, moving into a high-security "safe house" to prepare for the kill.
But in the world of black-ops, "safe" is a lie.
MI5 didn't recruit Mason to be their sniper. They recruited him to be their tethered goat. The house isn't a training ground-it's a trap designed to draw the terrorists out of the shadows and into the light.
With a killer closing in and his handlers watching from the sidelines, Mason realises he isn't the hunter. He's the bait. But they've forgotten one thing: You don't put a master marksman in a corner and expect him to play dead.