Thanks for your advice gents.
Just to clear up the floor..it's a suspended floor then.
A few pics from the install
Lounge floor
Gaps between ply
A section of the kitchen floor
Kitchen floor
The entire ground floor is a suspended floor. I had brick pillars made up to support the joists. I don't have any problems with tiles at the junction of rooms where they meet, the problems are in different places; some in the middle of say the lounge and in the case of the kitchen a few feet from a wall - completely random.
I own a builders merchant so I used top quality ply and didn't skimp in this area, it was mighty tempting but I resisted the lure of money savings in order for a better job.
So is it a case of getting my Stanley knife out and cutting away at some grout in order to pull a tile up? I guess it's the only way if I'm going to find out what is happening underneath .
I'm not in a position to call the tiler back, he made a complete hash of my bathroom mosaics and bathroom in general so I doubt I'll be seeing him again.