thanks chaps

my first thought was it could be a natural 'one off' type feature so i checked other stones in the area of which there isnt much left on site at all, sadly what remains of castercliff is in every dry stone wall in the area now but there are no cupmarks or signs of erosion in this way on any rocks nearby so i'm pretty sure its man made but then again i could be wrong.
As to its authenticy it was 2 feet down in a tree root so it could be old but not neccesarily contempory with Castercliff who knows? thanks for your comments.

peace :)