To top it off.... my passenger door was frozen shut when I went out just now!
My fiancee had to climb over my seat to get in (I made her take her shoes off!).
We went driving for a while to try and warm the car up. She couldn't get out and had to climb over my seat again. 2 minutes later, it just opened without a problem!
I wonder if the snow has got inside the door somehow?
It is -1.5 degrees C outside just now... but I wouldn't have thought the door should ever stick at that sort of temperature.