I am planning to help a frind bring the level of his hallway floor to match. He has a concrete floor that steps down about 100mm for about 1 meter before the front door. This is where the old porch used to be and is subsequently below the level of the DPC.
Questions:
1) I presume I would...
You probably have green grade chipboard flooring 18 or 22mm. I had exactly the same problem. Turned out to be the nails lifting slightly and allowing movement. Basically, I doubled up next to the nails using screws. This solved the problem for me. Allowed me to tile over without movement.