How funny. I just renovated (on a budget) my small kitchen and added exactly the slate look in tiles you have, though they are in fact porcelain. And I placed them throughout the house for continuity. The tiler asked if I was after a retro hippy look but it doesn't look like that at all. My cupboards were the same colour as yours and I utilised the bones of the kitchen and had the doors and Benchtops changed only. I also have plenty of light in the kitchen but chose white doors and as I was on a budget I chose granite look Benchtops, they are very serviceable and it looks great. The blue backsplash was the colour lift and here you could choose any colour to your liking. Basically the kitchen ended up with white cupboards and appliances other than cooktop, with the stainless cooktop and coloured splash back being the focal point. The paint colour are: whisper white walls, integra splashback, white ceiling and for increased contrast I used the same white above the splashback on the rear wall. I would consider painting out the brick but leaving the texture.

I work as a colourist and the easiest method for a upmarket look is to choose an exact colour to match one of the colours in the tile. I would take the tile to a paint shop and get the colourist it match exactly one of the greys in the tile. Make the depth of the colour a light version. All white walls are on the way out. Lappatto tiles usually have some texture as they are semi polished. This is the latest look in tiles as they give a shine without the maintenance of polished porcelain which had been in. They are usually less slippery than satin or polished tiles depending on the brand.

We had a similar situation with our feature wall bathroom tiles. We chose 600 tiles for floor/walls and feature tile and wanted the grout lines to all line up around the room. However, the feature tile was measuring at 595 and the grout lines looked wrong. I was told that tiles sizes are ‘nominal’ and variations can exist in styles and even batches. We addressed the problem by off-centering the feature tiles, meaning that the horizontal grout lines on the feature wall tiles met half way through the regular tile so it looked deliberate, rather than a mistake. Does that make sense? Would that be a possibility for you? The problem with the tiles being 2mm out is that with every grout line you add another 2mm out so by the end of the row of tiles the discrepancy can be quite obvious.

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.