Initially I just wanted the upstairs bedroom exterior (non cavity) solid wall levelled as it's bowed and unlevelled but most people have recommended just insulating. So after giving this a lot of thought and considering options for fitting insulated plasterboard I have decided the best way forward is to use 2x1 treated battens fixed at 60cm centres across the wall (with dpc behind) because this wall is all over the place and out of level top to bottom and across. With the battens I can easily shim pack out and get them level whereas any adhesive (dot and dab or foam) won't be suitable as too many hollow voids along the wall and harder to level the boards. I will then fill with 25mm PIR Celotex (or other brand) between the battens and foil tape any joins or gaps. Then screw on foil backed vapour plasterboard to provide that extra vapour control layer (I heard celotex foil already provides vapour barrier). Essential the stud wall approach here. Will seal any plasterboard joins between boards, sockets and along perimeter with Everbuild AC50 sealant and adhesive. Other option was to fix on 38mm insulated plasterboard directly on top the battens but then I'd lose that inch floor space because of the batten depth. Does anyone see any potential problems with this approach?