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Many thanks, Dogsbody; most helpful.
Yes, I guess the complexity comes from common law origins surrounded by statute (Prescriptions Act 1832, Right to Light Act 1959); a bit like the contract of employment in employment law, but that sometimes allows too much creativity in interpretation to judges - lawyers who used to make their living pursuing similar absurdities, hence my anxieties.
I cannot find any evidence of R2L applying to defend hypothetical future building, but neither can I find anything specifically blocking it.
Planning process is complex. We originally planned to develop in a different direction, without any implications for neighbouring farm land, secured planning permission, but rapidly hit an underground water cistern which we could not find an affordable way round. So we went for the present route, were told we could not treat it as an amendment and would have to make a fresh application. By that stage we had foundation trenches all over and two builders who we don't want to lay off just before Christmas. So we are proceeding whilst awaiting planning permission. We notified the neighbouring farmer as a courtesy and she brought in the Building Surveyor who sniffs a settlement fee.
We will see. In the meantime, thanks for your suggestions, particularly about teasing out his true intentions.