What's wrong with the central locking on my 2001 Ford Mondeo Estate?

I have a problem with the central locking on my 2001 (Y) Mk3, Mondeo Estate 1.8 Zetec.
The remote works fine when unlocking, but not when locking. It gradually got worse, sometimes it would lock, and sometimes it wouldn't. It now won't lock at all on the remote. If I try it locks then immediately unlocks again.
If I use the key in the door the old fashioned way, this works OK and sets the deadlocks and alarm.
Also when inside the car, if I try to lock all doors by using the driver's door lock, they all lock exceot for the rear driver's side door!
HELP!!
What is likely to be wrong, and how easy is it to fix?