Sorry ant. you can also uplift your chipboard floors and put wedges on the joints to level. Then re- install 25mm plywood over the top. If you have stud walls on top you need to cut boards out from under them and if bathroom is fitted on top you will need to remove this as well including skirting boards etc. The levelling compound is for an easier method around which i guess you have chosen as already tried this once. 1 other thing is if you have a floating floor you can not just put wooden wedges underneath. Bit more involved than that. If you do have a floating floor you need to leave a 5mm gap around the edge in the ply wood + levelling compound and the tiles