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Thanks jaffacake, that's helpful, I hadn't realised that. Anyway...
So in fact, she could then do what I said in my scenario i.e. punch a member of the public's lights out, then dust her hands off and stroll off to continue her walkabout - and she the law can't touch her? The police officers in attendance would just have to smile indulgently, shake their heads and arrest the victim if they retaliate? She could mug, burgle, rob, kidnap and murder people left right & centre, defecate in public, drop litter and poke police officers in the eye while screaming abuse at them - all with complete immunity?