Hello, I am a solicitor with over 15 years experience. I will try to help you with this.
I am afraid that it is your responsibility to cut back the hedge to the boundary line if he doesn't do it. If you were to do it yourself then technically you are supposed to offer him the trimmings before disposing of them your self. You have no right to invoice him for the cost of cutting the hedge. I do not think that he would have any redress against you if his hedge suffered as result of your cutting. I am sorry to hear that your neighbour has been so unpleasant but there is probably no legal redress.