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Hello, my name is Jim and I am a qualified lawyer happy to help you today.
You would need to involve a surveyor to deal with this - I would not recommend you draw up a boundary line yourself. So if you locate a suitable surveyor, ask the neighbours to bear half of the cost of the surveyor. That should sort the problem and if it does not, you may have to explore mediation as a form of alternative dispute but that takes both to agree to participate for it to work. You can say to the neighbours you are not a property expert and that a surveyor needs to be involved - remind them if they took this to court then the court would expect such a report to be obtained in any event.
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They may not but like I say their court action will fail (if they do issue a claim) without a surveyor's report
Yes I can only think they want more space on their land as you say. The boundary line needs to be as per the plans otherwise you can sue / apply for an injunction against them. The surveyor (being independent) should resolve this anyway