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Looking at your photo (zoom, enhance) the damage looks like internal condensation like the above, just a lot more of it, at a later stage. Looks like there's some black mould or similar below the velux and just above the rad, which (as I understand it) is a sign of internal condensation damp rather than external penetrating. Willing to bet they've had a wet atmosphere inside that they haven't tried to control through ventilation. Or a (stuck record) dehumidifier.
Maybe.
Howard
At a glance, your parents one is in better nick.