not a circle as such, but did you see that Dan Cruickshank programme about Bath?
you know that famous bunch of grand houses in Bath, built in a big circle (I think by the same guy who was involved in a similar bunch of houses in Edinburgh's New Town)? I can't remember the name of them sorry, but I know they're on the itinerary of all the open-top bus tours (Guide Friday?)
well apparently that circular formation has exactly the same diameter as Stonehenge!
Cheers
Andy S