Not long after I got my car the remote locking stopped working on the drivers door. It can only be unlocked now by putting the key in the door.

All of the other doors unlock perfectly by pushing the button on the key. What part do I need to replace?

Sounds like your door lock mechanism. Just double check your wiring loom between the door though.

SWMBO's Seat does the same. I've not looked yet but apparently corrosion on the connectors on the multiconnector that plugs into the lock unit is another possible cause.