Having considered cavity socks, BCO officer would now be happy for me to use masterboard or similar, slotting it into the insulation running up the internal wall and sitting it on the outer wall. The question is do I cut it so I can sit the soffitt board alongside it and seal the wall at the top with silicone, or do I sit the soffitt board below the masterboard? I assume going up and over the gable wall is straighforward. I also need to ensure it is protected agains weather and moisture ingress. Any ideas or comments?