Tenants removing a structure put up by them.
Three years ago, my uncle and his wife decided to emigrate and sell their house. We agreed (in front of witnesses) that they could put up a log cabin on my property (at their expense) whilst their paperwork is being done. The agreement was that they would live here, rent free for a period no longer that three years, after which the cabin would become ours. They would only contribute towards lights and water. The three years period has lapsed and I have approached them to start paying rent. They now deny that they ever entered into any agreement and want to leave and remove that cabin and all its fixtures from the property.
Please advise what my legal recourse is on this matter.