This happened to my parents some years ago. They got an affidavit from previous owners to confirm the boundary, they also got the 'owner' of the adjoining land to confirm.
Then they discovered that legal owner of the land was in fact the supposed owners long dead parents. They got an affidavit from each of the deceased's children. It took about 9 months if I remember correctly.
Maybe try to find a solicitor who has experience and settle in for a long campaign. You will get there eventually.