Hagar, topps are no longer permitted to recommend tradesman nor are they to endorse or approve any.
A good, qualified tiler would not recommend 6mm ply. This is half the issue with our trade these days. Gas needs gas safe, electricians part p, and potentially plumbers part h in the future. Unless customers of the future fully vet who they employ or we get some kind of accreditation anyone can call themselves a tiler. And by all means the concept is simple. But what about weights? Substrates, materials, expansion? A tiler should understand all aspects of their trade and not just know how to spread adhesive and grout.
A joiner can not answer if the floor ill be ok to accept tiles in my opinion.
Chipboard is relatively weak, and is naturally flawed in construction as far as a substrate for tiles. If its not water damaged then its not supported adequately. My advice would be to get rid altogether, fit extra support noggins and reboard with ply. Then overboard with your 6mm hardie backer board. Glued with adhesive and screwed as per installation instructions. You will also need to prime the rear and edges of your ply sheet.
Fair play to you wanting to take it on yourself, but you may as well take the free advice and do the job correctly. Good luck