"Cropredy finds one of these 'dead-straight' lines and tracks it for a distance of, say, 1km. Inserts own markers at 100 metre intervals or so. "
Dashed good idea. It's probably redundant to say so, but if no blindfolds are to be used then the line ought to extend over the brow of a hill... ;)
Here's a funny thing BTW, a Yuletide teaser.
Leylines, so far as I know, never go up cliffs. I wonder why?....