Hello my name is ***** ***** I will help you with this.
What is it you want to achieve please?
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My name is ***** ***** I shall do my best to help you.
The fact that it is your neighbours boundary to maintain does NOT mean that you can force him to replace the fence I am afraid - it depends on the actual wording of the deeds
If you read through the deeds you are looking for anything that actually says that the boundary has to be maintained and includes an Undertaking to that effect - if it does then this may be enforceable.
However it may be more practical - and cheaper to simply replace the fence yourself.
Please ask if you need further details