Howdo Mr E.
It's an intriguing looking stone, my first impression was of a field clearance with the stone bearing plough scars, however the basin looks interesting although it could be that this area was below the reach of the plough. Hard to tell but it's worth dropping your local archaeologist a line.

I have a copy of Derbishires 1872 excavation report of the axe preparation site at Ehenside Tarn that I could send you, it includes an illustration of a grinding stone. Unfortunately I have lost your address, if you drop me a line to my name at hotmail dot com I fire it off to you along with another nice paper on shap.
cheers
fitz