My friend walked out the pub to find a group of nine beating on his mate. He tried to intervene, peacefully (especially given the size of the group). The group choked him, cuffed him, beat him, pepper sprayed him. and...arrested him. Though he didn't know it and they had nothing identifying themselves as such, nor had they said anything. The group beating his mate were plainclothes police. They charged him with assaulting the police, and said the he choked and beat them. (All nine of them?) Does he have any chance at all under Criminal Law?