I agree with Nick’s comments – I’d be checking the state of the underfloor with a view to installing a vapour barrier and also checking where the extract fan is ducted.
I’m assuming an uninsulated house? Or is there ceiling insulation?

I don’t think I’d recommend installing vents in the cavity space or a solar extractor fan from the cavity at this stage – it is most likely that it’s an internal moisture problem – which should be addressed first as it will be making the living conditions less healthy as well.