In actual fact you could overlay ply on top of the tiles then re tile but not advisable as damp could and probably will acumulate between ply and existing tiles. For best results you should remove all the tiles, then if the walls are in a bad state you should coat them in a DPM then clad before tiling, although ply is okay there is a product on the market called aqua panel that is specifically designed for this purpose is very easy to fit, cut etc and comes in various sizes. Oh and take the advice previously giving and get rid of that tiler!!
If you need to find a tradesperson to get your job done, please try our local search below,
or if you are doing it yourself you can find suppliers local to you.
Select the supplier or trade you require, enter your location to begin your search.
Please select a service and enter a location to continue...
Are you a trade or supplier? You can create your listing free at DIYnot Local