We are having our kitchen redone due to problems with our suspended flooring. As a result we have put a concrete floor down. We have just had our tiler start on the job - laying 40m2 of porcelain tiles (60*60) We have just inspected some of the tiles that were laid and have noticed that a couple of them sound hollow in places and when we look around the edges of the tiles, you can see gaps between the base of the tile and concrete flooring. Please see pictures attached. *We also have underfloor heating which had been left on by the builders before the tiler started the job, not sure if any relevance to the above? Is this normal or do we have a problem?!