Like his character Colonel Mustard in the fourth season of
Cluedo (1990), Grantham was a convicted murderer. While he was in the army, he shot West German taxi driver Felix Reese in Osnabrück, Lower Saxony on December 3, 1966, in the back of the head while attempting to rob him. He was convicted of murder in 1967 and sentenced to life imprisonment but only served ten years. While at Leyhill Prison in Gloucestershire, he met
Louise Jameson who was working as a prison visitor in her spare time. She persuaded him to become an actor.
