Where the signpost points to Kemyal there is a house, park opposite and look through the field gate - the stone is in the middle of this field. The approach to the gate is very muddy so be careful. I spoke to the farmer at Kemyal Drea farm about stones in the area, and he said there were a lot of scratch posts but they were grey granite and new looking.
Taken from the Monument Class Description for 'Standing Stones', available on the EH site:
"Standing stones might be confused with cattle rubbing stones and still remain usually in the centre of fields, such as at Swingate, Cornwall, although it is possible that some cattle rubbing stones are actually reused standing stones."
From that, if you believe EH, it appears that this is in fact a rubbing stone rather than ancient menhir?