There isn't a maximum height as such, but if it is over 2m tall and evergreen it could qualify as an offense of the 'Anti-Social Behaviour Act 2003'.
'Part 8 of the Anti-Social Behaviour Act 2003 means a hedge can become an offense. The way this act is written is difficult to understand and open to some interpretation. There is no general requirement that all hedges should be kept below a certain height. For a hedge to qualify as anti social it must 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). Common misconceptions with this one are - It is not illegal to plant leylandii, and it is not illegal to have a hedge more than 2m high.'
You would need to make a complaint to your local authority and in order for them to pursue this complaint they will ask for an administration fee. Media URL: http://www.gardening-forum.com/forum/content.php?4-Hedge-cutting
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