Hi there. Not sure as to whether anyone out there has suffered this problem, but I've got the cure.
It manifests itself as a damp musty smell when just using the blower but seems to disappear when starting the aircon. The smell is caused by stagnant moisture in the system after using the aircon in humid weather. The cure is as follows:
Remove the cover from the pollen filter, and remove the filter. Next place some cloths over the internal air vents and switch on the blower to full speed. Then from outside the car spray Dettol disinfectant in aerosol form into the system from where the pollen filter was removed. This will kill the mould spores and any resident bacteria in the system. I used almost half a can which completely cured the problem. This product can be obtained from Asda and retails at around £1.79 per can. Well worth the money. Finally remove the cloths from the interior, refit the pollen filter and cover. Problem solved.
Regards. Len.