Andy,
I won't have time to prepare a 'proper' talk as I will be on the road most of tomorrow, but I have a selection of portal-type photos from the Outer Hebrides which give a feel for the range of sites there - happy to fill 15/20 mins if needed? There are a couple which may be rock art - or may be natural.
Something is not right. This message is just to keep things from messing up down the road