Speedwell

Speedwell was leading the investigation into the murder of exiled Time Lords Doctor Valentine and Professor Antenor in London when the Doctor arrived to conduct his own investigation. Together, the Doctor and Speedwell identified and killed the murderers - vampires Nessican and Cain. The Doctor located the transmitter used by Nessican to contact his employer General Tannis, modified it to create a computer virus that would affect Canisian weaponry, and asked Speedwell to pass it on to Brigadier Lethbridge-Stewart, at which Speedwell revealed that the Brigadier was his superior and that he himself was not a policeman but held the rank of Lieutenant Colonel - presumably as a member of UNIT. Speedwell and the Brigadier later co-ordinated the defence of Earth against Canisian invasion. (WC: Death Comes to Time)