thatbloke your right, I was going to suggest that, farrow and ... are well over priced. Undercoat the area that is giving bother, sand it and then apply the rest.
wow well done very scary working with old builds. I had to find solid ground once for the harth; i'm not a tall man but I ended up shoulder hight with the floor before I found it.
use toggle bolts they work fine with plasterboard and I'm sure your pic's are not very heavy. You have to make a small hole 10mm or slightly more to fit the toggle bolt through. If you hit studding just screw into that with a normal screw.
If you want to find studding in the wall it is...
Everything is ready to go but the cast iron fire front is a bit odd, there are the usual two wholes at the top and none at the bottom. Instead on each side there are two scew holes 20mm in.
My main point is has anyone ever drilled into a "fire back" I mean the sides of it and is it brittle...
I would never paint on new skim unless it has been dried for at least a 2 weeks. I too agree the plaster has not been dried out right.
This is the fault of your painter and he should be back to sort the problem out!!
I had the same problem when using pva on new plaster but at least I fixed...