We had a spookily similar experience with Greycroft - http://www.heritageaction.org/?page=heritagealerts_greycroft
http://www.heritageaction.org/?page=theheritagejournal&id=32

Can't remember, think we had a bit of action from English Heritage in the end, talking to the farmer about it. If they're joint guardians it might be worth approaching them. Vehicle tracks are the big issue, they shouldn't be treated as nothing, surely?