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Ventilation is key. Try opening the windows religiously for an hour each day (I know, I know, it sounds too simple but it will make a huge difference, whatever the weather). If there is a chimney breast and it’s boarded over, make sure there is an airbrick there (and that that there is a vent at the top), or that there still is airflow through it if it hasn’t. Adding insulation (internally or externally) needs to be done very carefully, as you’ll be changing the point at which condensation will occur and without understanding the physics of it you can cause a lot of new problems.