New plaster and cause of damp?

[Jn46]

4 weeks ago had some replastering done in bed and bathroom. House is mid terrace 1920s.
Looked to be dried out nicely so I put mist coat of dulux super Matt (as instructions, 3 paint to 1 water). Bulk of work was on rear south facing external walls in both rooms, where a lot of the plaster had blown and I'd hacked it off the wall, back to bare brick in parts.
Pretty sure plasterer PVAd then built up with Bonding, before multifinish skim.
Noticed damp patches coming through in bathroom yesterday that appeared dry previously.
Last two days have been warmer and damp. Bathroom is unheated at the moment, but well ventilated as there's a 20cm hole around bath waste that goes into roof void of kitchen single storey extension, so you can feel good draught coming through.
Some of the damp patches are on top of the old chimney stack. This was removed by previous owners on ground floor years ago, so starts at floor level first floor, then terminates about 80cm above floor level in loft conversion bathroom above. There are no air vents at all.

Questions:
-could the damp that's appeared be residual that's now coming through despite appearing dry previously?
-hydroscopic salt issue?
-should plasterer have used gypsum bonding (bedroom seems fine)?
-will it improve over time with vents to stack and some heating?

Suggestions?

Thanks

P.s got baby two due in 2.5 wks so any noisy stuff needs to be done before if poss!

 

[motorbiking]

Is this a 9" solid wall? It could be condensation

 

[Jn46]

Yes. Double skin London brick, painted pebble dash render on exterior.

 

[motorbiking]

I'd get up in the loft and check that water isn't tracking down. it could be that someone has broken something or rain is getting in.

Its a bit late now, but if I was hacking back to brick, I'd have installed inulated plaster board and just got the plasterer to tape and skim.

 

[Jn46]

No loft left as its a dormer conversion. Another finished bathroom above. Top of chimney stack was closed off and has been tiled. No signs of damp or water ingress up here, although I'm not going to pull the tiles off to look at this stage. Tiles or grout don't feel damp. Can't get on the roof easily. Unless it's a recent deterioration of materials that's caused a leak, I would have noticed it over last 14 months the lofts been done.
Pretty sure it's either salt sucking up moisture or condensation migrating from inside chimney stack. As stated the room itself is well ventilated at the moment so don't think it's general household condensation settling on cold spots.
Will try venting top and bottom of stack and getting rad connected in next few weeks to see if that improves it. Really don't want to take the new plaster down...

 

[motorbiking]

Any chance that the bathroom plumbing is leaking and the water tracking down the lowest point in the floor?

I had almost identical staining to the second image, which turned out to be a leaky overflow pipe.