My approach would be to re-lay 9mm ply screwed every 6"sq with a minnimum of 1"8 pozi screws.
Apply a coat of PVA primer and fix tiles using butech tile to wood (thats my favourite at the moment) or a SPF adhesive with a 9mm notched trowel. I also prefer to grout porcelain with butech floor grout as it spreads so uch easier over much larger areas.
Hope this is of some help
