Thanks Carpenter...
-the windows are sitting on granite sills and the granite sill suface is farely rugged. There are few small gaps when the window is sitting on the sill. The people fitting the windows filled any gaps between window and sill with silicon. I'd still like to cover the base of the window by a couple of mm with sand/cement mix to ensure all gaps are sealed. Aside from problems if we replace the window, will this cause any other problems?
-all outter facing walls will be slabbed with Kingspan. This will cover the rigid insulation point you made. Correct?