Thanks NickfromWales that sounds like a good idea. Unfortunately for me (and I was trying to avoid explaining in detail) I have already built the walls. after finishing the front, I sat down to look at my handiwork and came to the same conclusion, I'd just made a huge cold bridge. So now I was thinking, what if I cut back 100mm off the ends of the internal skin and inserted / extended the long wall insulation. Do you think that would get through building control?