Great stuff - Thanks for the advice - Much appreciated  

I am taking out a section of non-load bearing wall between two bedrooms.
When we bought the property, the concrete floors were all replaced (red ash issue) and as a result, the floor in one room is approximately 12-15mm higher that the other.
As this is going to be an enlarged master bedroom, into which we want to put sliderobes, I need to level the floors.
I am guessing that floor levelling compound can't be put down that thick?
Would it be ok to put down marine ply to take up the bulk of the difference, then apply the floor levelling over that?
Any help appreciated!