Can I first of all say what a great and informative website you have here.
I carried out work at a property around a year ago close to junction 27 of the M1, there was a problem on a south facing wall at the first floor, the area was formally a chimney breast (now disused and capped at the roof) although the chimney breast from ground to the roof is still in situ, either side of the breast is two alcove type insets on the same wall run.
This was heavely salted up, I treated it with a acetic acid from the company in your damp projects this Ive purchased before and not had any complaints, today I did, I was told by the owner that the salts have returned on new plaster work.
The roof was a new one when I carried out the work so Im discounting penetrating damp, is it at all possible that debris within the chimney breast i.e. soot deposits? maybe because it was not swept prior to capping off and moisture is ingressing horizontally along the walls, remember this is first floor not ground floor and the main problem is salting up.
Any advice on this would be most appreciated