Tiling on upstairs kitchen floor

Why is the board 1.2 X 1.2M ? They should come 8 X 4 feet and then there are less joints. Your tiler should have pointed this out.

Do you know that there's different thickness of mag boards?

Op I doubt that the magna board the shop told you about is practical for the job. The magna boards I've seen are very rigid and are only used as a waterproofing as that's the reason for them but there are not sufficient as an "anti crack" solution as your tiler suggested. It's a very good idea to use this matting that the tiler advised as it's upstairs and the existing plywood is sufficient as a base for the floor.

It's a slacker more so of the shop and their rep to suggest another type of board - have they explained whether it's an "anti cracking" feature or not?

BTW it'd be a good idea to upload a link to the "magna board" you have been told about so we can see exactly what type it is.

Another point not mentioned by either the tiler or the shop & rep is whether the tiler is going to use any tanking on the surface of the bathroom walls and floors before tiling. This is recommended nowadays by all decent tilers after the floor and walls are prepared but before tiling starts as it waterproofs under the tiles to prevent water leaking through into the walls and floors if any future cracks occur in the tiles and grout.

Iamhere I am a tiling contractor ( 37 years in the trade). If your tiler has suggested the flexible mat then why not do this, and it will not raise the floor height to much.

But if you are sheeting the floor What I would suggest having not seen your job, is sheet the floor in 6mm board as you did say its over plywood. Clean every bit of old adhesive from the ply first so as there are no bumps after the floor is sheeted. Do not bother with any of the boards from tile shops because in most cases it will cost a lot more and reason for the small size is for the DIY fellow to fit into his car. Ring a few builders merchants to check pricing . The boards may be called Magna or Multi-pro or tile backer board and even cement board . There can be up to 40% difference in price between merchants for the boards. The 6mm board will be perfectly good over the plywood for tiling, and the reason I suggest 6mm instead of 12mm is so the floor will not be raised to high at the doors. You dont want a feeling underfoot of steping up to the kitchen. The boards need to be screwed down not nailed , and to prevent the screws from going through the board, screws must be used in conjunction with metal disc washers.