C19

Department C19 (C19 for short) was the British government's liaison with UNIT, an international organisation. (MA: Who Killed Kennedy)

History
C19 pre-dated UNIT and started in the late 1950s. Following the Shoreditch Incident of 1963, it took over responsibility for the ICMG which handled that event. It maintained "the Vault" where it kept such things as de-activated Dalek shells and, after its de-activation, the sentient computer WOTAN. It used these to try and develop new military technologies and other, grotesque experiments such as infusing Humans with Auton characteristics. In the early 1970s, Martyn Townsend maintained it. (MA: The Scales of Injustice)

From the early 1970s (MA: The Scales of Injustice) to as late as 1989, Sir John Sudbury ran the department (PDA: Business Unusual). Circa 1981, Sudbury asked the Doctor to investigate the disappearance of a Concorde aircraft. (DW: Time-Flight)

Behind the Scenes
C19 and Sir John Sudbury first gets a mention in Time-Flight. The reference first gets first completley explained in Who Killed Kennedy by David Bishop.