Defender retro fit central locking problem

Hi, I'm in the middle of fitting a cheap ebay central locking kit to the defender.
The rear doors are nice and easy. On the front I have a problem. The locking mechanism only works manually by pressing the door handle button on the outside of the door while pushing down the catch on the inside. I've looked at disconnecting the button mechanism, but then the door won't unlock.
The defender is a 1990 model. Do I need a lock from a more modern defender for it to work?

Thanks for any ideas.