Tricky Dicky wrote:

Coming back to the tiles you will have difficulty keeping the grout
lines aligned because of the extra grout line between the 150mm
tiles. We tried it with some 150mm tiles and a band of 75mm tiles
between them, after just a couple of tiles the grout lines were
hopelessly out of alignment.

I was going to use 6mm spacers for the floor and 3mm for the wall, but
following Tim's comment that he hadn't noticed, and your confirmation
that it'll easily slip out of alignment, I'll try not to be too bothered
about it ...