If the subfloor is solid then just overboard with 6mm cement boards or insulation boards....why would you use 6mm ply? Apart from trying to save a few quid...you ever cut a sheet of 6mm ply and seen how easy the layers come apart .So If the floor boards or chipboard are solid there is no difference between 6mm or 18mm ply. If the 6 is glued and screwed every 100?