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, intending to use the Vault's equipment for his own end (MA: The Scales of Injustice)

C19 also had responsibility for the Glasshouse, a medical facility for soldiers who had nervous breakdowns as a result of alien encounters. (MA: Who Killed Kennedy)

Shortly before the arrival of the Doctor, C19 was compromised by both the Master and Townsend, both the Master and Townend's agents infiltrating via the Glasshouse medical facility. Both had differing agendas, but Townsend would assist in the Master's attempts to drive off investigative journalist James Stevens. These infiltrations led to C19 having an antagonistic relationship with UNIT, becoming a threatening agency towards civilians and anyone who'd been involved in UNIT operations, and attempting to assasinate both civilians and the nominal C19 commanders. Both would be eventually revealed and driven out. (MA: The Scales of Injustice) (MA: Who Killed Kennedy)

Dodo Chaplet was both brainwashed and murdered by C19, as part of one of the Master's plans. (MA: Who Killed Kennedy)

From the early 1970s (MA: The Scales of Injustice) to as late as 1989 (after C19 had been cleaned up), 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); years later, he brought in the Doctor to investigate SeneNet. (PDA: Business Unusual)

C19 was closed down in 1993. (PDA: Instruments of Darkness)

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.