The solution to the rear caliper conundrum was to purchase a shiny new pair of calipers from Rally Design. I contemplated getting the VAG hydramech rear calipers but they are not a direct swap and would require a change to the mounting and potentially a different sized disc. Given that the weight saving by using VAG calipers was only approx 400g (each side) I decided to keep it simple and stick with the ones from the Ford Sierra (and eat less pies)