Ye thats the only reason i had that sig on there to give people confidence in what i say. It dont do any thing for me as i dont get work of this site. Anyway. You only need to use 25mm ply if replacing chipboard . You can use this method and then pack your joists to get level. Take it you have stud walls and bathroom fitted on top tho, so can be alot of work to take chipboard up. If your going with levelling compound you can get away with 6mm ply if doing a 5mm+ screed and your chipboard subfloor is stable without bounce. If you have bounce in the floor you may need noggins put in and thicker plywood. Thing is you have to take up chipboard to put extra noggins in, so may as well put down the 25mm ply if doing this. If floor is stable just go with 6mm ply and compound. You can put down 12mm to be on safe side , but this is normally over floor boards.