There is rarely an obligation upon the land owner to maintain a fence or wall to a particular standard, it is the owners property and can treat it as such. There are some points of note however:

There may be a restrictive covenant requiring the maintenance of a fence. Details of such a covenant would be found within the Title Register and Title Deeds

If the hedge trespasses or causes a nuisance or injury then there are actions available. You should seek the advice of a barrister in such an instance.

Relevant Documents:

Title Register

Title Plan

Title Deeds

See Also:

Erecting a Fence

Fence Ownership

What is a Restrictive Covenant

Boundary dispute