Deller was a scientist whom Brigadier Alistair Gordon Lethbridge-Stewart once defeated. He was not British.
He was capable of making extremely sophisticated robotic duplicates of specific human beings. After the Brigadier defeated him, Deller found some means of sending one of his robots back in Time. He created a duplicate of the Brigadier's aide of fifteen years, Captain Summerfield, and sent him back to the Brigadier's first day at HQ, with a bomb disguised as a tea machine. He also sent the older version of the Brigadier back in time to meet his younger self so as to create a diversion. However, the two Brigadiers realised they were being played and identified the Summerfield duplicate before he could enact his explosive scheme (though not before he had killed the real Summerfield). (AUDIO: Memories of Tomorrow)