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finding any trustworthy contractors
My builder is a good place to start as all reviews on the site come as a result of winning work through the site. But always ask for references and check them, also ask to see their public liability insurance for small jobs, for big jobs a companies house check on the directors of a company is a good idea, don't go with anyone who has a string of bankruptcies.
Is that a pipe the runs down the side of the architrave in the first picture?
It's basically limited to this one, fully internal wall, which is the junction of the two reception rooms (knocked through) and the entrance hall, although it's started to creep to the other side of the door frame.
The water main is on the other side of the hall and CH pipes come down from the ceiling in trunking, not on this wall though.
I originally thought it might be residual ground humidity being forced up through the wall by the, now impervious, slab.
Like you say, I should get a few professional opinions on it, though I'm out sure how to go about finding any trustworthy contractors.