I have a 50m2 floor to tile in polished porcelain mixed substrate 21m2 of rough screeded sand and cement and 29m2 of 22mm osb over plyed with 9mm marine ply to bring it up level with the screeded flooring ,electric under tile heating supplied and fitted by us
Arrived on site today and the wooden part of the floor had been screwed down every 8" with 25mm screws and the osb had been screwed to the joists with 40mm screws,only trouble is they were the cheap black plasterboard screws, we spent the rest of today taking up the ply wood and osb ,sealing the back and edges of the ply then rescrewing the chipboard and ply down with 50mm wood screws ,turbo golds from screwfix, we primed the rough screed with bal apd primer then levelled it with fibre reinforced leveller, we used 5 boxes of screws and went out and bought a makita 18v li ion impact driver as our cordless screwdrivers were not up to the job ,tomorrow its ufh day and hopefully get a few courses of tiles down, pictures to follow

Nice day there Gary,handy when it's priced right.
I'm on a 40m2 floor since yesterday,2 sizes of porcelain! near done so tomorrow for sure now.Nice 70m2 polished porcelain next,floors all levelled for me