That's always a tricky one. I've never really found a clear solution to this problem, only that you are within your rights to cut the hedge back to the fence line but not over it as this would be considered tresspassing. If you have asked your neighbor to politely cut it back and still can't come to some agreement then you can try complaining to your local authority. If the hedge is considered a breach of the Anti-Social Behaviour Act 2003 it can become an offense.
To qualify for this it would need to be made up of a line of two or more evergreen or semi-evergreen trees or shrubs. It must be over 2 metres high. It must be to some degree, a barrier to light or access. Because of its height, it must be adversely affecting the complainant's reasonable enjoyment of their domestic property (either their home or garden).
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Description: contains more 'ins and outs' of the legal issues and more, to do with cutting hedges