My back door is locked shut and I can't turn the key - it feels like the lock mechanism is jammed tight - I can get the key in and turn in the direction that would lock it, but won't move a millimetre un the direction to unlock. It's a mortice lock (with a chubb type key?) and the door is timer clad in aluminium. Any ideas how to get it open so I can replace the mechanism? Cheers. Chris.