Usual type gearbox, lift handle to throw bolts and lock. Lifting the handle works fine and all the bolts engage freely without binding but the lock cylinder is starting to get difficult to turn to lock the door. If I pull the handle up sharply (but not with any further travel) the cylinder turns easily.
I'm assuming that there are no adjustments* possible to the actual gearbox and it's just a case of replacing it?
*Does the same thing if the door is open, so I don't think it's the door alignment.