Have had the exact same problem myself. we managed to get hold of some stuff - concrete remover - from a builders merchant. pretty strong stuff, try this. but make sure your tiles are pretty robust before you start. and also make sure the kids are out of the way! We (royal we of course) used this concrete remover together with chisel & / or wallpaper scraper. seems most of it is coming off. Scourers also will work, i guses with this stuff. I did a few tiles with just plain old elbow grease & scourer and it took me all morning! Good luck!