If the retailing wall forms part of the fence then the law says it is a fence and you are both liable to pay 50% each.
If it is not part of the fence and it is separate then the party that needs the wall is liable and that would be your neighbour as their land is being returned.
You also have to look at what is causing the damage. If is it from your neighbours land then they can be held 100% liable irrespective if the fence is part of the wall.
Here is some information.
You need to get an independent builder to come and have a look and give you a report on the cause of the fence failing before you approach the neighbour.
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