Gordon Craig
From Tardis Wiki, the free Doctor Who reference
Gordon Craig stood in for William Hartnell as the First Doctor during the recording of the third episode of The Tenth Planet, as he was too ill to work. He had sent a telegram to the production team informing them of this on the Monday before the episode was due to be recorded. (DWM 214)
He also had uncredited roles as an extra in The Smugglers (DWM 321) and The Highlanders. (DWM 292)