What order to start bathroom suite from scratch ???

[ChrisR]

Nah. Use the proper stuff and you won't get water under the tiles - which would be a disaster.
6mm ply is on the thin side, 9-12 normally recommenced. If you really want you can replace the boards with 19-22mm ply to avoid building the height on top of your existing boards.

 

[muffking]

Just the job.
Does it need to be treated or should I just go for a good seal?

I'm only concerned really as a few months back my 3 year old left the sink taps running and it overflowed onto the laminate floor which I had sealed, the water ran out to the hallway and got soaked up by the carpet. She should know better now, but I'd like to cover my @rse just in case.