I’m afraid I agree with your own research, it’s unlikely the freeholder is obliged to get involved here. The freeholder is an innocent party.
There is a rather weak argument that the freeholder is allowing nuisance and therefore is in breach of the lease but to be honest it’s not something that I would be going with.
However notwithstanding that, the situation is absolutely straightforward. The amount of land is actually irrelevant but it’s useful to know the scale of the problem.
Assuming that the amount of land which has been “taken” is measurable and there is no dispute over the boundary, then it’s a straightforward application to court for an injunction to make the neighbour move the fence.
It is trespass.
You may find that a strongly worded letter from a solicitor threatening a court application and an application for costs, may focus the mind without actually the need to get to court.
Check your house insurance to see if you have legal expenses cover that would pay for the legal cost of taking this to court.
I would not waste time with lots of correspondence, one letter and then a court application.
This is of course assuming there is no dispute over whether theoretical pencil line of the boundary lies. There is an argument over that, then you are going to need a boundary surveyors report to support your application.
I would start with the solicitors letter.
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