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What’s the pointing like on the exterior wall? We once had a damp patch on our Victorian end terrace halfway up the first floor level, that was eventually traced to a badly repointed section. It looked ok, but it was hard cement between soft bricks over lime mortar and the rain was running behind it until it got to a bit that was ok and just hung around there and eventually found its way inside.
no wall ties in this wall and that's paint not lining paper
I have been in the loft and there doesn't appear to be anything and also surely it would be at the top of the wall as well if it was coming from above?