Cheers for the responses guys. The car in question is a 1999 VW Passat Estate.
Will check the pollen filter tomorrow, didn’t think to do that actually! Fortunately it’s bloody easy to change on my car so will only take 5 minutes.
So far, I have been getting the car toasty hot on every journey, then opening the windows a smidge to let the humid air out. Obviously leaving some wet riding kit in for a day didn’t help! But that’s out now, and it was like it before…
No signs of water ingress into the car, or flood damage etc. Bought the car from a guy who lived out in the sticks, in a hilly area, so not likely to have been in any floods.
Cheers again, will see if the above will get me anywhere…