C19

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

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)

While officially, its mission was to liaise with UNIT and maintain certain facilities such as the Glasshouse, C19 also had a number of "black" projects.

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 or Cyberman characteristics.

History
C19 pre-dated UNIT and started in the late 1950s. Following the Shoreditch Incident of 1963, it took over responsibility for the Intrusion Countermeasures Group (ICMG) which handled that event. Group Captain Ian Gilmore, formerly of the ICMG, headed C19.

Rachel Jensen, also formerly of the ICMG, was the chief scientific adviser and was responsible for expanding on Judson's work with the ULTIMA Machine.

In the early 1970s, Martyn Townsend ran the Vault, intending to use its equipment for his own end (MA: The Scales of Injustice)

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 SenéNet. (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.