Windows going in shortly so need to figure out best and possibly easiest solution to insulating above the windows i.e. underside of dpc. Will be smart plying out the cavity(300mm) and there is a 150mm high gap on internal wall side down to 50mm on external wall side, so am thinking of getting beads blown in after windows are fitted and ply in place. Bit of a faf cutting Dritherm and trying to keep it in place before ply goes on, any suggestions?

Drill a 25mm hole in the ply internally and blow it in from there.