HA HA! Sometimes my stupidity astounds me! You're right, I just needed to pull the arms hard, as they had corroded and were really stiff. What is it with my and corroded locks?!
I'm surprised that they use such a low-security lock on something like that though. I'd have expected at least a pin tumbler with some spool pins or something. I know these things are just low-cost deterents, but someone could pick it with 2 screwdrivers in about 20 seconds! Seems strange.