Hiya Andy, I dunno which thread, though I'd be interested to read it too, if you should find it.

The ones here in Northumberland seem to be connected with grouse shooting. As I understand it, the heather is burnt off to promote fresh growth, which in turn, attracts the birds.

I saw some evidence of dodgy burning on Doddington moor recently, which came a bit too close to some rock art. Like the cairns you mention, it appeared that the fire missed the carvings more by chance than design.

Fire ain't good for old rocks.