The three stones referred to... while not modern... aren't prehistoric either unfortunately. The single stone in the photograph [removed] is a Medieval boundary marker known as the Moat Stone, while the pair of stones are more than likely old gateposts, there are wall foundation stones along this alignment which can be found after a bit of poking around. Again. that's not to say the stones are modern..... Gateham Grange, within very casual spitting distance of the stones, was part of the important Anglo-Saxon manor of Alstonefield, and the area was farmed at least from the 11th Century onwards.