Cut them down to the ground and plant a new hedge. We had quite a few black poplar that were leaning towards the neighbours sheds, more and more as bad winds came, (he was rubbing his hands saying he would get new sheds soon when they fell) so got the tree surgeon in and had them cut down! A lot higher than 20 foot, We chopped them up and logged them.
Also same neighbour is an excellent hedge layer around here and he said you cannot make a hedge out of a tree they want to go straight up so don't waste your time.
Hope this helps.