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I'm not sure if I've quite got your plan but is there a soil pipe running down that small bit of exterior wall?
I (well fortunately downstairs mainly) had an issue where the soil pipe had corroded and flushing it would splash water on the wall and eventually caused damp on the inside.
Edit: just seen those pictures and looks like there is.
aggi
stevo_com
There's perhaps about 6 inches of exterior wall to the left of the window, the really dark bit will be interior (onto the cloakroom toilet which is tiled so can't see much there).