I've been wondering about the suggestions of it being a radio nav aid. The only circular type of beacon I can think of would be a VOR beacon but these look quite different and I don't think quite as wide. I wonder if this could be an earlier type of the same thing which has now been abandoned hence no access required etc. On what appears to be the circular track in the bottom left quadrant there are regular radial features crossing the track (a little like railway sleepers). Perhaps these are shadows of short arial masts?
Definitely a fun little puzzle to ponder on. Thanks Jim, it makes sitting out here in the North Sea in a howling gale a little more interesting
Martin (Martin + Lynn = Mady)
PS: Just noticed something else. If you go to multimap
http://www.multimap.com/map/photo.cgi?clie...=700&height=410
You may notice that there appears to be the beginning of a track leading from the Shawbury airfield in the direction of the circle which stops abruptly at the edge of the field which contains the eastern half of the circle. Perhaps this was indeed the access road which, no longer necessary, has given way to the agriculture.