Another thought I am now having is that the lead was previously tucked into the wall. Where the brick was sliced, this would have created an easy path for water to travel down into this corner from further up.
I think I just need to get this damp meter down so I can see if it is changing when rain arrives, this would help! Stuart, they are interesting pics. So the water coming in could have stopped after new render etc but the stain would still come. Sospan, I am so tempted to start hacking away at the wall, the only thing that is stopping me is waiting for the damp meter to arrive
This was an inside wall, so there wasn't really a problem with penetrating damp. What happened was that the salts migtrated through the wet dot and dab when it was first installed and through the boards to the surface. These type of salts attract moisture from condensation even at a relatively low R/H, so the boards remain damp in those spots most of the time. Hygroscopic salts often cause problems around chimney breasts at the top, or sometimes at lower levels on walls with higher external ground levels. I'm not saying this the case for you though, the photo was more for interest. Tracking down the cause of dampness can often need a site visit if there are no obvious causes. In the area where I live the majority of builders are now using lime render and plaster on older properties where there is a damp problem as this seems to manage the problem better.
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