K11 Central Locking Issue

Hi Guys,

I have just rescued an unloved 1999 (T) K11 1.0 3 door Auto which hasn't been running for around 4 years.
In general, considering it was basically left to rot, it's in reasonable condition and seems to be running reasonably well. However, there is one annoying problem which I am sure you guys will be able to help out with.

The car has manual central locking (ie, via the door locks - no remote fobs).
The problem is that we can lock the car from the outside via the drivers door lock but can't get back in again. The external passenger lock turns both ways but doesn't do anything - either lock or unlock.
The internal door locks don't unlock the doors once locked (presumably deadlocked).
The only way back in is to climb through the boot and switch on the ignition (which presumably releases the deadlocking) and then either of the internal door handles can be used to unlock and open the car via the central locking.
The central locking appears to operate normally from inside once the ignition has been switched on and then off again.

I would be grateful if anyone can offer any help with this.