I finally bought a dehumidifier and WOW!
We all know Britain is very humid and muggy in the autumn/winter days.
My bedroom (in particular) and other rooms suffer from seasonal (edit) rising damp caused by condensation and build up of mould due to a lack of insulation which will be getting sorted in spring next year hopefully.
The dehumidifier came 2 hours ago, opened all the doors upstairs and let her run and just like that, the musty odour has almost disappeared.
It'll be down to 55% relative humidity in the next hour or two which is the recommended I believe.
Costs me about £1.00 - £1.50 to run 24hours a day. Can't believe I waited this long to get one.