Thanks for your replies. when checking from inside and outside the damp appears to be directly below the whole base of the stack. The curve of the top of the wall to the ceiling (think it's called "vaulted") makes it difficult to be sure , but that where it seems to be.
I live about 80 miles...
My daughter has asked me for help (as daughters do), but I'm not too good with damp problems, so hope someone here can help.
It is a spare bedroom so very seldom is heated, it has no fireplace but may have had because the chimney stack has 2 pots.The damp patches seem to be the same width as...