The Quiet Professional. The Useful Spy, #5

Sir Harold calls Miller in for a meeting. Asks how fit he feels because the next assignment is going to be gruelling. Information has come to light that at the SAS barracks a rogue element are taking justice into their own hands. A Major with a few of his men are secretly fixing problems as they see it. Miller will rejoin the SAS at his former rank and infiltrate the group. If necessary eliminate with violent aggression! His covert back story is that the manslaughter charge and prison, was all a front and that he has been under-cover in Iran identifying targets for a future attack by the USA.
Now back in barracks he feels a need to be part of something - with this the Major recruits him into the group. Then things start to go badly wrong not just for Miller, but the reputation of the regiment. Miller decides this solution lays outside official channels. Sir Harold agrees - no oversight - no quarter given.