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Thank you for your query.
I presume the fence is owned by the neighbour and is rotting in their area of land. So If the question you are asking is: How can I make my neighbour repair or replace his fence? then my answer is that you most likely cannot force her to spend money on her fence if she doesn't want to. It is, after all, her fence and if she wants to let it rot away then that is her choice.Don't forget that there is no general obligation in law that requires him to fence her boundaries, so you cannot depend on the forces of law to make him change her mind.
Hope this helps.
You could leave the old fence exactly where it is and erect your own fence right alongside of your neighbour's fence. So there are now two fences, one on your land and one on your neighbour's land. The boundary, being a line of no thickness, would then run between the two fences even if those fences are touching each other. But you can not legally do anything to her fence.
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