R v Williams (Gladstone) [1984] 78 Cr. App. R. 276

Victim saw X rob a woman and restrained X. Defendant, who hadn’t seen the robbery, thought that Victim was unlawfully assaulting X (from the fact that Victim lied to Defendant about being a policeman and, when Defendant asked for his warrant card, obviously Victim couldn’t produce it). Defendant tried to help X by assaulting Victim.

The judge ruled that Defendant’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.