I'd contact the archaeology service at the County Council in the first instance - they don't publish direct email addresses throguh the website but try http://www1.dorsetforyou.com/Portal/ContactUs.nsf/Contact?OpenForm .
Another tack if that fails to stir any action is to contact someone high up in the Cabinet of both the County Council and local district council and talk about far right infiltration in to middle England but this might back fire if the police decide to take a closer interest in the site - any visitor will get quizzed until they are blue in the face.
G