ey up Lubin -
I like the cup-marked stone you've posted. You say there are 3 on Dartmoor - have you got any images of the others. And do you have any references describing them? Who first found 'em? When? Where? Why? What!? I'm a bittova cup&ring man miself.

One thing of serious note: if these cup-markings are lejit, there's gonna be more of 'em up there. Check stones around the nearest cairn-field and settlement. I'd be very surprised indeed if the 3 you say are here are the only ones.