My guess is that warm moist air out of the house is condensing up in the chimney. It happens a lot inside lofts so I'm guessing it might up the chimney. It's only a guess mind.
It's definitely rain water penetrating, almost certainly through the brickwork. Note that the internal wall of the bedroom becomes the outside wall above roof level. Original (now hidden) felt roof is well mortared into the brickwork, and the box gutter is similarly sealed into the bricks a further 3 courses above this.
Would I be better off simply treating the masonry with some kind of waterproofing solution? I have found the following on the web:
Liquid Plastics K501
UltraProof
NPA Clearshield
And good old Thompson's Water Seal (which has a reassuring "Very High VOC")
All claim to penetrate deep into the substrate, and allow the wall to breath so trapped moisture can escape. On the subject of trapped moisture, would a water-based product be better - I think that dampness would stop VOC's penetrating (dry surface is specified in the instructions), e.g. Thompson's One Coat Water Seal Ultra (water-based) "can be applied to dry and damp (not saturated) surfaces."
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