Removing wallpaper revealed blown plaster?

[tacittosa]

We've just moved into a terraced mid-1800s house and have begun redecorating.

When removing some wallpaper from a lower-ground floor room, the paper pulled away a layer of paint, and in some places some plaster too.
On closer inspection it looked like some areas of the plaster had blown, and crumbled away fairly easily.
The revealed remaining plaster looks like it is potentially slightly damp in some areas, as it's darker than other places, and feels slightly cold to the touch - but there was no signs of any mold to the (mostly white) wallpaper or paint.
Tapping on the plaster below, above, and to the side of this patch it seems solid and still attached to the wall.

What should be my next steps? I'm assuming if it is an issue with damp there's no point repairing the plaster with an easifill type product until the damp is sorted?
I don't know enough about plastering to know what the surface is that has been revealed now the top plaster skim has crumbled off?

I'm slightly unsure where the damp would be coming from as the rest of the wall seems fine, but being a lower ground floor room I suppose it would be prone to it..

Any thoughts much appreciated!

 

[foxhole]

Party wall? Chimney wall? Exterior wall?

 

[tacittosa]

Sorry, should have said - it's a party wall, in the alcove to the left of a blocked up chimney.

Though it is a lower ground floor room, there is a small yard to the exterior wall, with stairs up to street level. It is in the corner that the stairs are at to the external wall - here's a pic from before the wallpaper was removed for clarity:

 

[Wayners]

That's looks like recent multi finish plaster over the old plaster possibly lime plaster?

House that's old within lime plaster need breathable paint or damp will get trapped.

Maybe get damp survey or others specialist to look at problem and provide spec to revolve

 

[Bullet-Proof_Biscuit]

Is it near the blocked chimney breast? Or against another wall?
Chimney breast needs ventilation. Is the chimney pot still open? Is it cavity wall?

 

[tell80]

tacittosa,
are you still there, come back if you still need help?

 

[tacittosa]

Thanks for replies!
I ended up chipping off the loose plaster and flaking paint, sealing with dilute PVA , easifilling the area and then papering with a thick lining paper, and am going to get someone in to try and track down the source of any dampness that might have caused it in the first place.