DIY Doctor UK DIY Forums

I've had a kitchen remodelled and as part of this the old floorboards were removed, celotex insulation installed between the joists and a new floor laid using floor grade chipboard. I want to tile the floor but I hear conflicting reports about why it is essential to lay ply boards or tile backer boards before tiling over a wood floor. The argument behind this is usually that the floor could move but if I've had a new floor laid and this has been recently and thoroughly nailed down surely this is now as stable as ply over an old floor ? Hope someone can answer this because I then intend to lay an engineered wooden floor over the rest of the ground floor but I'd then have to lay ply or something of similar thickness over the rest of the ground floor just to keep the surface level at the same height throughout and the work and cost involved would be large.
Thanks