Try to find his autobiography - "The Long Banana Skin". Good read, very funny.
IIRC he grew up within earshot of Hythe ranges; and at one point was the fastest draw in Britain - and by draw, I don't mean "pull out pistol and fire a blank" like the muppets who dress up as cowboys and line-dance, I mean, "pull out pistol, fire a live round, and hit the target".
GB Senior (no slouch with a pistol himself) once mentioned seeing him draw, fire, and hit six plates with a revolver in some unfeasibly fast time.
Bentine was hated by Milligan for being good at everything he did. When they were recording the early goons they went to an RAF station where one of the pilots had won an olympic medal fencing for Poland. Knowing that Bentine fenced, Milligan set him up to fence with the Pole. Bentine won and very shortly afterwards had to leave the Goons because Milligan wouldn't have anything to do with him.
Bentine worked for Mi9 during war, his immediate boss was Airey-Neave. He also took part in the liberation of Belsen.
There is some suggestion that his father (a Peruvian) was also a spy, working for Britain.
Bentine was certainly regarded as a crack pistol shot, as well as being a yachtsman and general all round adventurer.
he had an abiding interest in the afterlife, and was well regarded in the psychic world.