Snow taken into the car on shoes then melts into carpets. Car then does not go far enough and heat up enough to dry out carpets. Condensation forms and freezes in this weather.

Have they got the heater control permanently on re-circ?

I have the same problem on my FN3 Civic. If the aircon is used in cold weather together with the heater on demist to keep the screen clear on short runs, when the car is stopped the aircon evaporator element is damp due to condensed water, sitting close by to the hot heater matrix. When the car is stopped this hot damp air rises up through the demister vents, condenses on the cold screen and then freezes on the inside of the screen if it's cold enough.

The solution is to turn off the aircon a mile or so before stopping to dry the evaporator element , but leave the heater on demist until stopped to keep the air flowing through the system.

It's also worth checking that the pollen filter element is dry, and that the aircon drain is clear and dripping clear water when the aircon is running.

Oddly, my wife's Mazda 2 does the same thing. It's happened a few times the last few weeks

Her journeys are 5 miles, which should be long enough for a 1.3 litre engine.

The heater is never on 're-circulate'

The AC is never used.

The carpets are not damp

Sometimes when her car is parked up, there's lots of condensation inside whereas I can't see this on any other car on our road.

What other reason could there be?

After my last service in late October last year, I noticed the inside of the windscreen had moisture every morning. Knowing a new pollen filter had been fitted that was the first place I looked. On the B class it is accessed via the grill between the windscreen wipers so very easy to access.

Opened the bonnet and looked at the cover which is about a foot long and has 3-4 clips top and bottom with a rubber seal to prevent water ingress. I could immediately see the bottom wasn't clipped in properly.

Took the cover off, reseated it and ensured all the seals were flush, the problem was solved.