"There'll be a 'third' one somewhere about."
There is! At Marlborough. And all three can be seen from The Sanctuary.
(Please disregard both those statements as we're in enough trouble as it is...)
1889 map....apart from "well" there appears to be no sign of it on here though barrows on Overton Hill are shown. Does any other map show it?? http://www.avebury-web.co.uk/silbaby.html