OK, pimping almost finished...
So, onto the basic mdf walls I added 1mm foampvc to the outer walls and added the aforementioned wallpaper. The window frames sit just above the wallpaper layer which I like, plasticard window sills and Slaters quoin for the corners. Some cutting of the corners we needed to allow the quoins to sit flush, which was miscalculated at some points, so quite a bit of green stuff needed there.
For the inside, I ordered a spare window set from the same company and used those to give extra detail inside. I also used 2mm foampvc to thicken the walls again. These are slightly above the window frames, which works better for the inside (for me anyway).
Stone pattern plasticard for the floor, plastic trim in the tower. Scratch built double doors. Quite pleased with it all so far. Just need to finish the top of the tower and then texture the interior walls. And then the roof...