You have potentially acquired an easement of support in respect of the land which it appears has been like that for more than 20 years. You would require that either under the Prescription Act if it is more than 20 years or under the doctrine of Lost Modern Grant if it has been since time immemorial.
You have not acquired an easement in respect of the fence because you say that it has only been never 13 years BUT if it replaced the previous fence in the same position, then the time the previous fence was in position and the current fence would be added together including time of previous owners. Here is some reading on easements of support:
You also have a potential claim, regardless of the above, in negligence you have something in place and their work is making it fall down.
If they want to move property, and they are selling their house, they are going to have to resolve this pretty quickly because a new buyer is unlikely to want to buy a property with this dispute in place. You need to ask them to confirm that they have advised the buyer of this dispute and told their solicitor and the agent and that if they don’t reply, you will be telling the agent and the solicitor of this issue with the property.
By perpetuating this problem, they are in effect, shooting themselves in the foot in respect of selling their house.
Can I clarify anything else for you?
I’m happy to answer any specific points arising from this.
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