Okay thank you, that's food for thought.
When it happened in January this year, I poured warm water onto the calipers which seemed to have very little effect. I had to wait for the ambient temp. outside to rise. When they unfroze, it was a little bit at a time, which to me would suggest that it wasn't the pads sticking to the disk? When I first went outside and released the handbrake, there would be no tension on the handle anywhere between fully off and where I had had it applied to over night. Gradually the 'tension point' would be lower and lower (5 clicks, 4 clicks, 3 clicks etc) until I could drive it.