Ah ... a local site that's probably not even for local people. If the field is directly over the road from a farmhouse then it would a fair guess that you should ask there. Although it does seem as if this may be fruitless.

I'm always uncomfortable with people saying they own our heritage. Although they may own* the land surely the monument should be seen to belong to all - as long as people are sensible and well-mannered about visiting.

*I won't even start about the evils of land ownership itself :-)