Dear Ste, Or anyone removing polystyrene tiles.
To remove tiles, use any scraper but once tiles are removed, then you have to remove adhesive under tiles. This can be easy if you are using a heat gun. I found by using a heat gun level where the tiles was and a scraper this would remove the adhesive but be extremely careful IF PLASTER GETS HOT IT WILL CRACK. Caution be careful not to burn your hands too. (From experience I've spent more time trying to find correct angle not to burn myself ;or plaster to cause it to crack). A normal scraper for clearing wall paper is okay but do not use cheap scrapers they won't take the heat and become useless.
Do not waste your time using drill with wire brush attachment ,it will only leave black marks and you'll be wasting your time.
Anthony Tovey, September 2010