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.

Tile selection can have a big impact on the cost of your bathroom reno. The cost of the tiles per sqm is one thing, but did you know that most tilers charge different rates for the "Type" of tile laid large format and or vitrified tiles will cost more to install These are a few hard and fast rules we give our clients when they are selecting tiles for a bathroom renovation 1 - opt for darker tiles and darker grout for the floor - as a wet area light coloured tiles on the ground are harder to keep clean 2 - If you are opting for a feature tile - let it be the hero. That means make the rest of the tiles plain - large format subway tiles give you enough texture without detracting from the feature 3 - always opt for square tiles on the ground - trying to get the correct falls using rectangular tiles will mean you will have extra cuts and the floor will look a mess. The second reason we opt for square format for the floor is because of subway tiles You see, back when everyone was opting for square tiles on walls, it was easy to install square on the floor and get all the grout lines to match. Now most people are opting for subway tiles, you can never get all the grout lines to match, therefore installing square means you dont have to match grout lines as they are different format. If you are installing your subway tiles vertically, then there is a chance you will still be able to get all grout lines to match if you have selected the same sized square format tile hope this makes sense I have added a link to a blog that explains more about tile selection

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