We have decided that it would be a good idea to put up a studwork partition and then attached the insulated plasterboard to it. The original plan was to attached the plasterboard to the damp proof membrane (that brown stuff), but we have decided that this is a better approach.
However, it does mean that insulated boards have to be put in the studwork. Just as well since we have some boards left over!
Originally, this gable end had suffered with damp. It had a partition that was insulated with rockwool. We have put a damp proof membrane on the wall and have then constructed a studwork partition. This will be insulated and then covered with plasterboard.