It seems that it's the deadlock part of the mechanism that is seizing up. Last night I had another go fiddling with the lock. I tapped the back of the lock (from inside the door) with a hammer whilst locking/unlocking the door, and hey-presto, the door is now open! I can lock/unlock the door fine on the normal lock, but if I deadlock it, it's hit and miss if the door will then re-open. Now I know how to get it open, that's not the end of the world. I'll get a new lock fitted at some point. I offer this advice in the hope that it will save someone else having their door mutilated by the garage trying to smash the lock off. Cheers, Mark