There are strict rules on Adverse possession.
You have to make an application once the 15 years are up and the owners of the land are not aware you have been using their land and you have improved it.
If you have not made an application with the land office to have the boundary changed then it is not going to be an easy task.
The link I have given you is very detailed on the law and what has to be done.
Having knowledge that their lad is being used by a third party does not give the third party the right to make it theres. It has to be improved and used without the knowledge and consent of the owner.
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