Our bathroom has existing tiles on a solid floor (well, solid enough - it's thick ply on top of 300 year old oak beams and it's not moved in the last 20 years!).  I'd like to lay some underfloor heatig on top of these tiles and then retile over the top.

Any problems with that?  Any concerns?  Do I need to clean the tiles first? With what?  Abrade them?

Thanks guys