What am I looking at and how shall i fix it?

[ree]

OP,
Its hard to clearly see your latest pics but the plaster/render should not be in contact with the solid floor - something to remember when you make good the defects is to keep the render about 50mm off the floor.
If you remove the plaster/render then please post a pic.

 

[frustin]

I removed the radiator and the gypsum mortar
.

 

[ree]

OP,
The plaster needs to be removed back to bare masonry - any existing DPM will then be exposed.
You might also be able to locate a DPC in the wall itself?

 

[frustin]

The plaster needs to be removed back to bare masonry - any existing DPM will then be exposed.

are you talking about the yellow coloured sandy stuff behind the gypsum plaster? i'm not sure how thick it is an i'm not sure what's behind it. I stuck my finger into that hole on the left and it's about 4inches deep. I think it's lime base coat, no idea what it's adhered to.

You might also be able to locate a DPC in the wall itself?

There's no DPC, this part of the wall is 300 years old.

 

[ree]

OP,
Well, based on what you say & no DPM being present then I would still attempt to cut back from floor contact?
Given the age you mention then its highly possible that the nib of wall is stone faced, and filled with random rubble?