I have cut away a damp part of my bathroom wall and removed all the ply and polystyrene. I have replacement polystyrene and ply but the section I am replacing has no battens behind, the polystyrene was stuck to the aluminium of the outer wall and then the ply was stuck to the polystyrene. I am having trouble sourcing an adhesive that will be fine for both the polystyrene to the aluminium and then the ply to the polystyrene. Any ideas much appreciated.