Tom
there's a paper called 'Pictish symbol stones: caught between prehistory and history' by Meggen Gondek, in the recently published book, Art as Metaphor: The Prehistoric Rock-art of Britain.
It's a great piece of work and a good read. As I read it I thought of you and your bullauns. It's well worth checking out as her methodology would probably work quite well with bullauns.

cheers
fitz