Okay, the switch round the barrel is for the factory fit alarm (only fitted from Jan 1990 onwards and will have 3 wires, brown, brown/white and brown/yellow). It is NOTHING to do with the central locking. Yes, the wires go back to the central locking module, but they do not work the locking system itself, more "piggyback" the factory alarm into the system.

You say it works from the switch inside fine, does this mean that all the doors lock fine?

You also say that it does not work from the key on the outside, does this mean that the central locking does not work, or that the key does not lock ANY door?

If the central locking is all fine from inside but only locks the drivers door from the outside, then I would suspect the mechanical latch itself is @ fault.

If operating the barrel does not lock any doors, then either the barrel is worn too much, or the lock is broken internally.

If the keys locks only the one door, then I would look @ the latch or the connecting rod as it would sound as if the latch is worn excessively and not moving the motor rod far enough to activate the central locking.

What happens when you try this from the other side? Do you:
a/ get the same problem
b/ all doors lock fine
c/ the central locking system "cycles"

It could still be the CDL module itself, but most likely it is the motor or latch. Wiring issues normally cause random "cycling" of the locks. If a master motor should lose its earth connection, then it would become a slave unit. This would cause the system to always "cycle" as these motors cannot operate on a single master circuit as they always require the second master to act as a confirmation.