OK, I just checked mine. There's no lock in the battery. The lock is attached to the frame, and only a pin goes into the battery case, so I can't help you. You'll have to call Cyclezee in Milton Keynes. He holds a lot of spares.
Another thing you can do if you can get the lock off the frame is dismantle it and remove all the tumblers, then you can operate it with any key or screwdriver. Often, you need the key to remove the lock, so if you don't have a key, you drill out the lock-body to remove it, but then you need a complete replacement.
Dismantling these locks is normally very simple, something like remover a circlip and slide out the lock-body. the tumblers will fall out then.