"Gunman who killed Pc will die in prison". Which is small consolation for the relatives of the murder victim.

What I would like to know is how someone sought for murder in his own country could get into Britain, live and work here for 7 years, and even get married, without being detected?

Don't bother trying to explain. I'm not that interested. I just want it prevented in future. 

Obviously, what is needed is a reliable and convenient means of identification employing biometrics (e.g. iris or finger scan; DNA profile).

The sooner we have it the better (The Need for a National Voluntary ID Databank).