Hello forgive me for sounding a bit thick but i have a replica victorian cast iron fireplace and a set of tiles for them, How exactly is the best way to fit them?? Every time I try either the gap is too small for them to slip in or too large and they keep slipping out. The fireplace is for decorative purposes only and is being fitted to a flat wall. Do the tiles have to be cemented at the back. I would be grateful for anyone's help as I have come very close to smashing the lot of them on the floor now in frustration!!