Screed floor over cavity

[Notaclue123]

I recently ripped up my carpet by the front door with the intention of laying some engineered wood. Underneath is a screed/concrete floor which, next to the door, had cracked and was loose, I've chiselled out all the loose stuff and have now exposed the cavity beneath the door (see pics). The cavity is filled with concrete/rubble just below the exterior DPC in some areas and above it in others. There is also a plastic DPM that runs from underneath the existing floor and reaches to about the inside edge of the cavity.

My first thought was to just fill it with a repair mortar and level it but I want to do it properly as I don't want to have to rip up the wooden floor 2 years down the line.

I've done some googling into similar problems and the advice is generally fill with weak mix concrete to 25mm below exterior DPC then run insulation over the top and then lay the screed on top. However there is no layer of insulation that I can see in my floor (built 1986), just 40mm of screed over what I think is a concrete base...??

Would it be adequate to just pour a concrete mix straight in to the cavity and level with the floor (which presumably was how it was done in the first place) or is it better to use something like a cavity closer to seal the cavity and then pour concrete on top? Do I need to lay a new DPM to bridge the gap over the cavity??

Also when repairing a floor like this should anything be used to tie the new and the old concrete together?

Any advice is greatly appreciated as to be honest I've pretty new to all this.
Thanks

 

[Tigercubrider]

I had something similar where a patio door had been added in place of a set of French windows, a dwarf wall on each side crudely hacked to floor level. The original floor was parquet and the original Dec was just bitumen

What i did was to excavate down a bit to remove loose stuff, adding a base of dry compacted sand before a dpc and concrete

I would probably scrape the debris and bring up the level to dpc
Then add a chunk of dpc that covers the gap with an overlap- if possible tuck it under the hall one?
Because I had it, I used a bitumen paint as well to aid the seal
Then a screed using the existing floor as a guide

One suggestion might be to add a piece of conduit in there should you need to run a cable side to side?