I can't help with Scottish engine rebuilds etc but I took mine to AAS in Newcastle to have the heads changed.
They did a little bit more than heads, as there was a problem found when they were taken off. (pic)
they stripped it completely and rebuilt with new pistons, cam, heads, clutch, intake and had it tuned afterwards.
I supplied the heads/cam package, from American Heritage Performance in USA, the car is bonkers now.