Not clear how serious is your leakage. Is it just seepage or water flowing thru?
Are you able to identify the exact location below the bathroom slab where the water is coming thru?
Is the leakage between the UPVC piping and the concrete slab?

If your house is still under DLP, you can get the developer to rectify the problem at no cost to you.

If you are replacing your bathroom tiles, better to put a layer of waterproof membrane. This should solve your leakage problem.

From you described, it looks like it is more of a leakage than seepage. You should remove the tiles in order to address the problem permanently.
If you are able to identify the exact location, then try removing the affected tiles and carry out the repair using hydro plug. Hydro plug is
cement based product for plugging holes in concrete and masonry structures, to prevent the passage of water.

They can be purchased from some hardware shops. I recall buying them from a hardware shop in Taman Megah - SS24, PJ. It looks like the normal Portland cement and comes in a can; in which you need to mix with water.

This post has been edited by kelvyn: Jul 23 2011, 10:17 PM

If that's the case, try removing the WC carefully so that not to damage it.
Remove the cement around the sanitary piping, etc. Remove all the loose cement. Then use hydro plug cement or sika non shrink grout to fill up all the gaps. Leave to the grout to set. Test again for leakage.