Floor repair over wall cavity

My house was built in 1968 and we have lived there since new. It has traditional 10inch cavity walls and concrete floor (on the ground floor). At the rear of the property the floor slab has dropped by about 5mm. This is not a recent thing, but happened very early in the buildings life and has been stable for many years.
My problem is that the floor over a door opening (over the inner course of brickwork) has not dropped and the resulting step makes carpet laying difficult and the thin concrete layer was starting to break up.
I have chipped away the concrete floor at the opening to reveal the inner course of bricks and the cavity. The top of this inner course appears to be coated with a hard grey resin-like material, which I think formed a dampness barrier (this is at the same level as the damp course and looks to have be put on with a trowel). This had adhered well to the brick and concrete but mostly came of when I chipped away the broken concrete. The cavity itself was filled with sand and rubble (now removed) to support the thin concrete above and appeared to be smeared with the same resin-like material as above, but this was very thin and week. Both the inner and outer courses of brick and the sand in the cavity are slightly damp to the touch.
My question is -
Should the sand and rubble in the cavity be removed? If so, what should be placed into the cavity to support the new concrete that will eventually bridge it?
What can I use to replace the "resin-like" material; it must act as a moisture proof barrier over the inner course of brick and whatever I use to bridge the cavity, plus it must adhere to brick and allow concrete to adhere to it?
I anyone can offer any advise I would be most grateful.
Regards to all
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