Of course it's not beyond the bounds of possibility that some one may *know* what happened. But even if that is so, they'd be wise to keep it to themselves, I think.

As for the polis, I don't think that a lack of action from them necessarily means that nothing 'serious' has happened; it could well be a case of "he said, she said", with little chance of a successful prosecution in court.

Without speculating on this particular case - what often happens, especially with drink related offences, is the "victim" tells the police he/she does not wish to press charges and does not want to provide a statement. Police then decide to look at alternative cases more worthy of their time

unless a criminal act (violence) has occurred then they must act themselves

That is incorrect, I know from experience

You're wrong on that one