Dave

hi kelly..
the plywood should be minimum of 12mm and of exterior grade.....( most adhesive comps say 15mm/18mm ) but on 99% of bathrooms 12 mm will do
it should be fixed with rust proof screws every 200/300 mm.
If a thin ply has been used then it is probably the sub-floor that is to blame especially if the floor was springy or in bad repair to start with.. i.e. loose boards etc..
WHAT TYPE OF ADHESIVE WAS USED... this should be a flexible type cement based product..suitable for tiling onto ply with..was the ply primed..the adhesive could be breaking away from the substrate and was the grout used a flexible one...all this questions need to asked.....as for the bathroom company saying its not covered in there guarantee then that's rubbish..stick to your guns and demand it is re-done properly......The tiler should never have fixed the tiles if he knew it was incorectly prepared for him..what else has he covered up that was prepared wrong.....get back on the phone and demand it is sorted or you are going to trade and standards......good luck..