Just out of interest; did you buy the house from the builder? I know it's been 8 years. But NHBC covers 10 years, it won't cover movement and settling damage of the property. Like cracks in plaster. This is what they would put the failed Silicon down to. BUT, i think it would be worth a shot trying to find out who did the tiling, installation and kick up a bit of a fuss over it not being right. You never know.