Discolouration/"damp" on chimney breast, alcove

[parakeet]

In my Victorian terraced house, the middle room chimney breast has this dark patch at the lower right...

FYI - in 2012/13, the house was renovated by a cowboy developer-owner. That involved removal of the front chimney stack serving the front room and this middle room. House was re-roofed over that.

We noticed the discolouration during our ownership.

See this 2016 photo, taken during works, which shows "tanking" solution applied to the affected part, as it was then...

My first photos, above, show the room newly decorated in October. Two things seem true:

* Despite a couple of coats of damp-focused stabilising primer, then two/three coats of Polished Pebble, the discolouration showed through within a week. Pretty disheartening.
* When you look at the tanking solution applied in 2016, the present wall seems to have been spared discolouration in that same, defined square shape - see the straight line of the top of the square. ie. The solution worked?

So...
- What's going on here?
- How should I address it?


I'll also note...

#2) It's not necessarily confined to that area of the breast, but potentially elsewhere on the right...

* In 2016, the same works were carried out to make-good areas of "damp"(?)-affected plaster on the right-most wall of the right alcove. Initially speculated to be seepage from a connected external wall.
* As of now, there is deterioration at the lower-left of the same alcove. I just discovered a Trivial Pursuit box all furred-up...

#3) I have several other "damp" concerns in this house, including another part of the same room, though I'll leave these out of the post about this specific area.


Thanks.

 

[^woody^]

Condensation damp within unvented flue.

Or flue salts reacting with the mortar or plaster.

 

[parakeet]

Condensation damp within unvented flue.

Or flue salts reacting with the mortar or plaster.

Thanks.

And "heck". So the only solution to condensation damp, I read, is ventilation top and bottom. We have neither.
Is there any chance this could be rising, or is it just impossible to tell?

Also, I wonder what might be a good option for leaving it essentially without the proper fix but trying the best to stop it manifesting on the surface. The tanking solution seemed to do okay in the section on which it was painted, but now the problem is evident around that. In my mind, I'm imagining applying tanking solution to a much wider area and then repainting, though I'm aware this would be merely masking the problem, and I guess there's a chance it just moves the water around to somewhere else.