Hmm, loads of really useful comments here. I don't think it's water ingress as the carpets are bone dry, that said the seal on the drivers door has started to peel away from the door a bit so that is probably not helping - I have been meaning to get that fixed since I bought the car, so it's about time I got it sorted. As for the windows being clean, I'm normally pretty good about this but I will give them a thorough clean over the weekend just in case. To be honest I think a damp pollen filter is the most likely culprit, I've had lots of cars with climate control and I often just leave the system on auto because I'm lazy like that, but I have never had one steam up from a cold start like my GTI does. I might just pop down to the dealer and get a new pollen filter in the morning - I take it it's pretty easy to find & replace?