Whether the landowner is doing it or not, the fact there's access means others will be attracted to do so, and perhaps on a very big scale one night.

Let's face it, we're the ones that think it might be something, and the various authorities can't/won't act so it's down to us.
We could write to the owner (or HA could if they're agreeable) politely explaining that we think it "could" be something ancient, and offering to help in some way if he was willing?