I take it the conifers are on your neighbours land and that you have just been "maintaining" them on your side where they have grown across the boundary. If your neighbour is not willing to come round and prune them back you are entitled to do this yourself (or get someone to do it for you). Legally you have to let the neighbour know you are going to do this and offer the cuttings back to the neighbour. You cannot chop them down, nor can you make the neighbour pay for getting the work done. Of course, if he was being neighbourly he would either do it himself or offer to pay for it to be done.