Nup, a good tiling job requires a good substrate. The best tiler in the world can't make a bad substrate look clean and good. No point on spending money on new tiles and a good tiler over an inferior base. Rip it all down, start again and get it right before tiling. Best of luck, Dr Retro Of Dr Retro House Calls

Have you priced having the whole lot re waterproofed and tiled? You are already doing the hardest section, it may not be that much more to do the lot. Otherwise agree with above match to the white tiles in the rest of the bathroom. But it will always look like a repair.

You may be lucky & the tiles may come off easily if just laid (we just did this in our shower). Worst case when they pull them off they may damage the membrane, in which case they will need to repair it, so it is just the time of the repair & drying, before they can relay the tile. If it's important to you, get them to fix it, as long as you can accept the worst case scenario of the extra time (that is what we did, and then the membrane was fine anyway). Good luck.

quite possibly, that was going to be my next solution! heading back there in a moment to check it out. the existing tiles have been layed quite nicely/flat so i dont think tiling over would be an issue