Repair base of built in cupboard

[JustAnotherJane]

Our house has some age (built early 1800s) and we want to renovate the built in cupboard that was built in to the alcove of an old fireplace (now blocked up).
The skirting that previous owners edged the bottom with looked rotten so wanted to get that out and it looks like lots of old / failed concrete has come down behind it.
How am I best tidying and sealing this?
Or is this a bigger job than I thought?

 

[blup]

Concrete on a plastic membrane.

Blup

 

[JustAnotherJane]

Phew. I can see a corner of plastic sticking out of one end. What about around the stones . Should we just pack in more mortar?

 

[blup]

That maybe ok if it is not compromised, just be careful taking the loose material out and you will see. I remember repairing a floor, the damp proof membrane was literally like cling film, and had worked for at least 30 years.

Blup

 

[conny]

Fill the mortar joints then lay a new membrane and concrete over level with existing floor?

 

[tell80]

OP,
You need to post pics showing the front of the chimney breast, the full opening of the fireplace, and skirting at the sides (cheeks) of the chimney breast.
What you call "failed concrete" is probably sand & cement render.
I suspect that you might have rising damp in the fireplace walls? Knocking off a little of the render/plaster seen in your pic # 410 could reveal more info?
Is the floor a solid floor?

 

[JustAnotherJane]

Will try to take more pics after work tomorrow. The fireplace is fully closed off and chimney capped off.

 

[tell80]

Then where exactly are we looking at in the first two pics? Context needed?
"alcove" means the space on either side of the chimney breast but I assumed that you were referring to the fireplace - "built in to the alcove of an old fireplace"?

How long have you lived in the house?
Do you have a survey report?