In reply to George Fisher:
I'm a solicitor with a great deal of experience of boundary disputes.
I'm afraid Siward is pretty much on the money. Trangia will also be along in a moment with some sensible observations.
The trouble with ADR and the various other methods of resolving these disputes is that, while they are splendid in theory, they can only be used if both parties agree, and since generally in order to have a boundary dispute at all you need one party to be a bit of an arse, they often can't be agreed upon.
If they can't be, then the only two methods of resolving the dispute really are either some form of litigation (as to which see Siward) or asking the Land Registry to determine the boundary. This last is supposedly a vanilla alternative to litigation; the trouble is that if it gets contentious the LR refer it to a LR Adjudicator, which is basically the same as a court.
It's not the finest feature of English jurisprudence, I'm afraid.
Still, if you want to email me pdfs of the two plans, a photo of the thing and an account of why you think you're right and what he's saying, I'll give you a ten-minute freebie with a largely worthless gut reaction if you like.
Better a bad settlement of such a dispute than a successful litigation, though.
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