You should start by contacting an estate agent who is experienced in valuing this type of land/property in your area. If you don't know any then contact RICS to get some names.
You should get a value from the agent & then you have a proper basis to negotiate from. The agent would, og course, negotiate for you if you wish. You will have to pay the agent a fee but should then be able to add that to the value - after all, it is entirely the fault of the trespasser that you are having to do this. Your legal costs should also be paid by them.