Took tiles off the bathroom window sill. Half way across (about 3 large tile lengths) the underlying plaster/mortar/stone (not sure what it is) started to crack. I managed to get the tiles off but some had about 1/2 to 1 inch of this stuff from underneath stuck to it. I managed to jigsaw puzzle the front edge back to gether but there are holes in the middle. What is the best way to repair this?