In case anyone else wants to know - I ended up being able to change it by, with the lock in the 'open' position rotate the the 'n' shaped upper part of the lock 90 degrees, to the other groove, press the top part of the lock into this, and simulataneously turn the dials to the new code you wish to use.
This can only work if you know the original code in order to open the lock in the first instance (!)