Collinson

Sergeant Collinson was a soldier under the command of General George Carrington. He led a gunfight against UNIT and spared Brigadier Alistair Gordon Lethbridge-Stewart's life.

Arrested by UNIT, Sergeant Collinson refused to answer the Brigadier's questions but was tricked into revealing that he was a soldier by the Third Doctor. He was freed by somebody who held a gun to Corporal Steve Champion's back as he fetched Sergeant Collinson a cup of tea. (TV: The Ambassadors of Death)

Behind the scenes
Collinson's name is never given in on-screen dialogue; his name is only given in the closing credits.