Is this it?
http://tinyurl.com/2wpxul (links to Google maps satellite view - I google earth'd from your instructions, and it looks like there's a stone in that field)
If so, the the ref is approx SX 034 545 - EDs will be able to change it.
Oh, and RotS says this stone was referenced as early as 1525, so if it's a rubbing stone, it's been there a while!