Well, my plan of using the Golf as a winter car has hit a snag. Each time I have been out to it first thing over the past few days, the drivers door won't open......

It's a five door with remote locking (Through an alarm.) First day, every other door opened but not the drivers-the button didnt even pop up. I pulled it up (It was very stiff) from inside via the back door, but the door still wouldnt open with either inside or outside handle. A couple of hours later, it was fine and opened normally. (Once the temperature had risen a bit.)
Yesterday, the car wouldnt unlock at all-it would try but then relock, presumably because the drivers mechanism was again frozen. Again, a couple of hours later it was fine.
This morning, same again. Drivers door wouldn't open. Pulled the button up from inside with no effect, but this time left the car unlocked. An hour later the door opened normally.
Now this problem is clearly temperature/ice related as the locking has been 100% fine up until these cold temperatures. I have no idea what the internal lock/latch mechanism looks like, so where should I start in trying to sort this out ?
Many thanks.