Hi - I live in a semi-detached property and there is an unused chimney stack on the outside of the building. Recently I have noticed a change in the paint work just below the ceiling in the landing. This discoloration looks like damp and is situated on the inside of the wall to the chimney.

It isn't condensation as the window in the landing is always open and I always keep the bathroom door closed.

The chimeny stack is looking a bit 'funky' as the paint looks bobbly and there is some moss on it. I'm putting the problem down to penetration. Please can someone give me some advice on how to deal with this. I'm guessing I'll need to strip the paint off the chimney stack, repoint any missing concrete? Is there a recommended product to seal the brickwork before I repaint? I'm I missing anything else?

Quick question is there an attic above the ceiling in the landing , if so i,d go up into the attic for a quick look to make sure there is no leak coming for above ...
If the water is not coming from above i,d say that the brick work needs re-pointing , its an easy job but not a nice job
as for special paint i,d imagine you could but damp proof paint but any outdoor/ waterproof paint should do