Mystery damp....

[teaonesugar]

damp1.jpg (142.84 KiB) Viewed 1247 times

damp 2.jpg (177.96 KiB) Viewed 1247 times

Looking for advice on what looks like a problem with dot and dab causing damp patches on an internal wall.
I bought my house nearly a year ago, noticed small signs of damp next to the stairs (paint going 'dusty') but under stairs there's no sign. It's gradually spread, larger blotches on the wall slowly joining up. There's a damp proof course in the wall, the other side is a covered passageway. These patches have appeared in the corner next to the front door (started about ¾ of the way up) and next to the stair but nowhere else. No pipework in that wall, or above it.
I'm completely stumped, but wondered if a temporary solution might be to just drill some holes to let the moisture out(?) while we save up for a damp specialist survey.
Any advice gratefully received, thanks.

[wes56]

Your photos are out of focus.
That gable wall has been dot and dabbed,is that right? How has the stair string been fixed to the wall?
Is the wall solid or cavity? Does it have cavity wall insulation if its a cabity wall?
Can you post photos showing the other side, the outside, of the wall at lower level? And a photo showing the covered roof of the passageway?