In reply to Tom Valentine:
If you ever visit the Westmorland Show (or indeed just the ground - it's just off the M6 at Crooklands nr Kendal) they have short sections of wall in most of the vernacular styles, including the "West Yorkshire" style.
I was thinking of the stone balancing chap as it happens. I can just imagine the "fun" they had trying to put the second or third course on!
I think we may be suffering from confirmation bias - I had a look at the walls either side of the lane as I walked up it this morning and those have "holes". Interestingly I build "solid" walls - I rebuilt the wall on one side of the garden and built the wall on the other side from new and they are solid.
Trangia: My back's fine thanks
Rigid Raider: Those walls at Coldwell are closer to the walls the Inca put up, all the stone is dressed, they're closer to mortared walls. Yes, it's gritstone. I too admire them as I ride past.