For almost 50 years MI5 had agents embedded at the BBC, vetting job candidates with the specific aim of weeding out prospective left-leaning employees. It was known as the “Christmas Tree” process, after the discreet symbol on a personnel file that would advise executives that a particular individual was to be blacklisted. The practice continued well into the 80s, and until a 1985 Observer exposé was denied at all levels. Perhaps this jars a little with the warm and fuzzy image of the BBC that has been bequeathed to us over the generations. This lovable national treasure, informed by the...