We have had black bits appearing on our upstairs ceilings and window frames since moving in 10 years ago. This is probably due to the double glazing having no vent strips along the top of the frame. In the winter, the windows largely remain closed (or at best on first latch as we don't want the cat falling out). Would definitely give the Aero 360's a go before contemplating more drastic measures. Also make sure you wipe any overnight condensation off interior window panes. That and wipe with a bleach solution as noted and periodically repaint. We also have a cold/slightly damp patch on one of the living room walls (only in the winter), no visible defect outside missing mortar or hole in the wall. Could be a gap in the insulation (if there's even any in the walls) but so long as it can be managed I'm reluctant to hand a builder a blank cheque to investigate, assuming we could even get one interested!