Victorian Fireplace hearth/subfloor renovation help

[Sismithin]

Hi all.
I’m in the process of laying a new 2 tile porcelain hearth for my new log burner and re pointing the back bricks.

I’ve dug out the old heart tiles and discovered someone before done a terrible half a job before me in laying half a subfloor.

As you can see the hearth had a 25% mix of original ash dirt subfloor left behind and then a newer self level screed which only fills half the hearth.

My question is how should I go about putting a DPM down and self level given what’s already there?
I need a level base to sit my new large tiles on but I don’t want ground moisture problems either in the wall or under the floor.
Any advice is greatly appreciated.

Thanks

 

[Sismithin]

This is before. I have a new fire being installed so wanted to do everthing properly.
New tiles are large 20mm thick porcelain slate style tiles/slabs.