My neighbour next-door-but-one has grown a Leylandii hedge on the boundary without the permission of my neighbour next door, whose garden is overgrown. Who is responsible for maintaining his half of the hedge: it’s currently 7 metres high and restricting light to my garden.
In the normal way of things your neighbour would cut the hedge that intrudes into his garden. But this wouldn’t necessarily reduce its height, and it sounds as though it’s not a concern to your immediate neighbour. You should obtain (or download) a booklet from the council about high hedges, which will tell you whether you are able to make a complaint depending on hedge height, orientation etc. But in order to use the high hedges legislation you will have to seek to resolve the matter with the hedge owner in the first place.