Give this to your neighbour:
Your neighbour refuses to repair his fence. Unless he is constrained to do so by a restrictive covenant then there is no obligation on your neighbour to fence his garden or to maintain a fence that already exists. There is also nothing to prevent you from erecting your own fence upon your own land adjacent to the boundary, even if the deeds do give ownership of and responsibility for the boundary to your neighbour. In fact, if you have security needs (for example, connected to your young children or your dogs) then you are under an obligation to cater for such security needs and in effect you must erect a fence.
There are two ways of proceeding.
1. Volunteer to meet the cost of replacing your neighbour's fence yourself. If your neighbour agrees, then you should make it clear that the fence is a gift and that the position of the boundary remains unchanged. It is best to record this in writing and keep a copy with the title deeds, so that no one in future thinks that the boundary has moved to your neighbour's face of the fence in keeping with the fact that it is you who paid for the fence.
2. Erect your own fence, on your own land, as close as possible to the existing boundary features (so that you don't later lose the land between your new fence and the true position of the boundary). Be careful not to damage any of your neighbour's property (fence, shrubs, etc) in the process. Also check that your new fence is no higher than the local planning bye laws permit.
Lots more about fences
here