Thanks for the pointers. DPJ, I understand the theory of what's happening, but I don't accept that there isn't a solution. There's people who live in much colder countries than us and they own cars, so there must be something out there!
I'm very wary of having doors that don't shut. I've had one that had delatched, but not open because it was frozen shut. Then when the car has warmed up, the door has come open because the ice has melted!
I have also had the doors frozen open, and I also use the brute force technique!!
I might try WD40 on a rag later, I just wanted to avoid WD40 because of the smell. I wonder if there is a similar lube that doesn't smell as bad.
And whoever uses hot water is a braver man than I am. A bit of frost holding a door or lock is one thing, full on ice blocks is an entirely different kettle of fish!