Thanks I've heard 25-30mm. That 5mm will make quite a difference, with the transition to the future carpeted room.10mm underlay and 12mm carpet. Ive then got 8mm to make up with carpet shims. Probably not a great finish as you'll see the slope and trip on it. Other option would be to screed the carpeted room's floor, but that will run into a small fortune I don't have.