I have a upvc door that the latch is operated by both handles , I want it so that the latch can only be opened from inside on the handle and would need the key from outside , at the moment the key does not operate the latch just the handles . Do I need a complete new lock unit or can it be setup to work as I want it too ? I don't know the make of the door but the lock has a G U and an arrow on it ?..