I've visited the cup marked stone and can tell you it isn't easy to find, So if you like i can supply you a 10 figure grid reference taken on my gps if that would help. I've noticed the grid reference isn't posted under the sites information which i'm assuming is deliberate?, So i wont post it here unless ok'd by the sites moderators, though i'd be happy to e-mail it you if you can supply an addy.
That would be great Harestone. I didn't get to go up onto the Ridgeway today but grid ref would be very useful on my next visit.
Like you said, I think the grid ref has been omitted in the past to protect the site and avoid hoards of people upsetting the landowner.
My email is on my profile page (changed it recently) by the way.