If he serrated the floor I would expect to see the serration marks beyond the tiles, there is one or 2 where this is not pressent and that's what initially drew my attention. + the shadow under the tile + the rather poor screeding. Jas, don't think I'm being piccy, I would have concerns about that floor if I where you. Time will tell how good the installation is mate but if I where you, I'd be lifting one of those tiles for further inspection to see!