Ha ha - I think you can probably work out the answer for yourself! I've just googled it and this is the first time I've heard about it. No, the plumber and tiler never even mentioned this.
I've had a very apologetic email back from the plumber this afternoon who has accepted full responsibility and wants nothing to do with the tiler again (apparently he met him in a pub a few weeks ago) grrr. It looks like he's taken a risk but then passed that risk onto us. He's determined to put things right, redo the floor tiles, fix the plastering and redo the edging and he doesn't want any money until we're happy with things. I think, legally, we have to give him an opportunity to amend things before we cancel his contract or take things further?
I've also had another look at the wall tiles, and although they were originally 300 x 600 all of the ones at the back wall of the shower have been cut to fit, so ended up at about 300 x 350 and on the side walls, they were either cut to fit (ended up about 300 by 250) or have had the shower wall screwed into them, which was a bit of a relief. Having said that, I've spoken to the extremely friendly and helpful Josh (thanks!) and I'm going to see what happens if you slide a tool between one of the tiles at the top to see how well attached it really is.
I'm glad the plumber isn't denying there's a problem or wanting to send his useless tiler back to us. Lots of lessons learned for us. Ugh. This whole job was supposed to have been finished almost a week ago.
Thanks for all the advice everyone, you've been fantastic.