Nigel
It's interesting that you bring up the Thornbro alignment.
The North Yorkshire antiquarians from Knox through Mortimer to the mighty Elgee seem pretty much in agreement that triple barrow alignments of the North York Moors reflect the handle of Charles or Carles Wain. There are at least 9 groups called 'The Three Howes'.
Elgee notes that barrow groups of between 6 and 7 monuments at Hugglegate Wold, Hulleys, Ebberston Low Moor, Carlton Bank and the ridge between Farndale and Bransdale also 'suggest the plough'.
Check this out
http://www.themodernantiquarian.com/post/44244