No exceptions? Gareth Williams and the ensuing ludicrous cover-up springs to mind. Let's not forget David Kelly, either. Sure, someone else may have killed them, and we'll likely never know the truth about it. But MI5 is authorized to commit crimes on UK territory, and everything about this remains hush. Interesting how a fair number of spy/ex-spy poisonings and deaths take place in Britain though. If they are telling the truth, then it means they're pretty incompetent, what with Russian hit-squads waltzing in and out all the time taking out whoever they like, without MI5 preventing it, or presenting conclusive evidence of it, after the fact. But that would be a very big "if".