The priority has been the same for hundreds of years: protect the Establishment from "subversives" threatening instability. This includes anyone leaking damaging true information. The grip seems to have loosened in recent years, partly due to overseas media and partly due to the security services genuinely having increasing panic over ISIS and Islamic radicalisation.

What is being concealed? Not just Zircon, but all the persistent allegations of sexual assault on children. A lot of the stories that have surfaced include elements of either the Metropolitan police or MI5 taking over the files and then "losing" them or silently shutting down the inquiry.