Have bought an apartment in an old 1950s block which has floorboards fixed to batons laid into a solid floor throughout. A few have been cut in the hall to retrofit pipes for the central heating and a gas fire. The living room adjacent has the same sub-flooring and is about 21m2 but is not level at all (you can almost see where it dips to the door threshold. Looking to finish with parquet floor so really need to sort this out. How should I go about levelling these rooms? My thinking is to screw/nail down ply throughout and then use this SLC https://www.screwfix.com/p/mapei-ultraplan-3240-self-levelling-compound-25kg/4959f over the top. I've seen 12-18mm ply suggested for under slc but can I get away with thinner if the floor below is solid? How should I fill the gaps where the pipes are? Any advice appreciated - have used the Mapei SLC once before and it was magic but this is a whole other ballgame.