Although the preceding advice is sound and will probably take care of your problem, you might actually have a problem with the underlayment for the tile installation. If it wasn't properly prepared using a cement board (assuming the underfloor isn't concrete) or other such backing, the tiles could have
uncoupled because of the flexing. Tile floors can't stand much flex in the floor. A new product I've used has provided great service for this problem without the need for all the work and expense of cement board. There are other brands, but I used Schluter-DITRA successfully on older floors. (
http://www.floorheating.ltd.uk/schluter.php ) It's a rolled product and easy to install. I would suppose it or other types are available in the U.K. (I'm in the U.S.), if that proves to be part of the problem.
Good luck!