V saw X rob a woman and restrained X. D, who hadn’t seen the robbery, thought that V was unlawfully assaulting X (from the fact that V lied to D about being a policeman and, when D asked for his warrant card, obviously V couldn’t produce it). D tried to help X by assaulting V. The judge ruled that D’s assault, if based on private defence, had to be judged on the circumstances as he believed them to be, provided they were based on reasonable grounds. CA said that “reasonable grounds” were irrelevant: the circumstances were to be judged subjectively and therefore the conviction was quashed.
