Having never heard of the grey wethers, could this be referring to the same stones?

"Unfortunately the Grey Wethers in the valley below Ashdown House were not turned to stone by Merlin. They are an unusually large collection of sarsen stones, perfectly natural though not necessarily native to this part of the country. Sarsens are believed to be erratic rocks, brought far across country by long gone glaciers. They were often revered by ancient man."

http://www.berkshirehistory.com/legends/george02.html

Hob