I just found this link:
Message Boards: Alibi
and there is this message:
Fri 15 May 2009, 2.43PM
Last of the Summer Wine
"The stones from the Episode "Welcome to Earth" do not exist so don't go Yorkshire trying to find them! They were put the BBC props debt on a field overlooking a lake which is off Choppards Road, off the Dunford Road heading out of Holmfirth towards Sheffield. The same field has been used a few times in the recent series."
Greg

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