Damp halfway down a corner

[Eryna]

Hello,

I'm back with more plaster questions.

I live in a top-floor flat that was converted in 1989. The house itself was built in 1879.
I've got some damp issues between an external wall (plaster on brickwork, supposedly, with some kind of shiny dark sheet in between) and a partition wall (plasterboard).
The weird thing is that this only affects the bottom half of the corner, up to the smaller stain a little further up. The darker patches of plaster that you can see on the second picture feel damp to the touch. The upper half of the corner looks and feels perfectly fine.
To my knowledge, there are no pipes in any of those walls. The kitchen is on the other side of the partition wall, but all pipes go straight into the floor, as far as I'm aware.
The wallpaper over the partition had a bit of mould on it, though not as much as I expected. The paint layer that was right under it came off on its own. There might have been what is called efflorescence on the exterior wall, but I can't really tell. No tape whatsoever, so you can see the gap.
Supposedly, there used to be a chimney on that side, if that changes anything.

This is what my walls used to look like:

(The previous owner tried to conceal it - but he picked the wrong paint. )

And this is what they look like now:

I was going to fill, tape and joint the entire corner, then skim the exterior wall with Toupret, but I'd like to understand where this might be coming from. If the roof was leaky, surely this would start from the upper corner?
I'm just afraid that this might all be for nothing, and the managing agent can be a real pain when it comes to flagging any problems to the freeholder. (When I contacted them about the cracks in my plaster, they just said to instruct a building company to check the integrity of the structure, which is absolutely not my responsibility.)
I don't have the means (or living space) to hire someone to take everything down and replaster it all...

Any idea or advice will be gratefully received.
Thank you for reading!

 

[Harry Bloomfield]

Any idea or advice will be gratefully received.
Thank you for reading!

Have you a photo of the other side of that wall?

 

[Eryna]

Hi Harry,

More or less - it's quite high up and there's another house not far away from it.

The waste pipes on the left come out of the eaves storage space, which is not part of my flat per se. They're on the right side of my exterior wall.
The boiler flue comes out of the kitchen. The boiler itself was installed close to the partition wall I was talking about.
So the damp corner is somewhere on the left of the flue on the above pictures, if that makes sense.

 

[Harry Bloomfield]

Mmm, it looks as if someone has had that wall skimmed, in an attempt to keep out moisture, and that skimming hasn't worked. Do your downstairs neighbours have a similar problem?

 

[Eryna]

I haven't thought to ask! I'll try and leave them a note.
The worst of the damp isn't even at skirting board level, which really adds to the mystery for me!

 

[Jahjahbinks]

maybe removing that paint will let the walls breathe for a while. I had a similar issue near a fridge and bookcase. removed the terrible paint and mold etc is gone after cleaning. I'll now mist and repaint.

also these management companies are terrible. my partner hasn't heard from there's since bought and there are MANY issues including a new roof needed and we've paid out of pocket 2x