UK Property Law
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Thanks for your question.
There is no strict legal process to decide who should approach him, it's more of a practical question. I would suggest that it would be more persuasive if both you and the freehold owner took action over the issue. It may be more likely to convince the owner of the adjoining property and it will ensure both you are your freehold owner are on the same page.
Try speaking informally to the owner encroaching and seeing if he is willing to comply, take along your evidence of the encroachment. If he does not accept it then write formally to him enclosing your evidence and requesting he cease the encroachment within 7 days.
In the final analysis it benefits the freehold owner because they would not wish to allow the encroacher any chance of claiming adverse possession over the land and for that reason they should join you in attempting to remove the encroachment.
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