This is an answer, not a question.
My UPVC door wouldnt lock in tempertures of -10, i couldnt lift the handle all the way up, an the key wouldnt turn- the solution is to heat the room the door is in, i put a fan heater in my porch for about 30 mins, the lock works perfectly, mind you dont put the heater to near the door, as the door will have the opposite effect and expand. Hope this helps. WD40 may help, as it doesn't freeze.

Dizzymoo, December 2010