How far over a fence can a neighbour prune a tree?

Here in the Village you have to get permission for everything !! It's a conservation area. The next door neighbours Oak tree was absolutely massive, blocking out the light etc.. They were given permission to crown it this year. On the other hand there is an awful Leylandi tree ( however you spell it) it keeps snapping telephone wires and so far they will not give permission to get it lopped . So stupid as it really looks horrible ( this is in the front garden )