Despite all doors being closed, the door ajar light still lights up. I'm read this is controlled by the microswitch in the door lock, but doesn't anyone know how to determine which lock is throwing up the fault? If I could narrow it down to one door then at least I know what to fix / replace?

If you get yourself set up with FORScan it should be able to tell you the status of all the doors, hatch and bonnet, i.e. whether they are showing up as being open or closed.

That definitely worked on the MK2.5 models and I would think it would be the same on your model. 

Try putting the GEM in service mode, the indicators and a tone should sound with each door being opened or closed.

l    Switch off the ignition
l    Switch off all other electrical consumers
l    Apply the handbrake
l    Shift to neutral
l    Close the doors.

Activate service mode
 
Proceed as follows to activate service mode:
 
l    PRESS and HOLD the switch of the heated rear window
l    SWITCH ON the ignition.
l    RELEASE the switch of the heated rear window

A signal sounds and the turn signal lamps illuminate to indicate that service mode has been successfully activated.

Note: Ensure the windscreen wiper is in the OFF position
If the alarm is activated (in vehicles fitted with an anti-theft alarm system), service mode cannot be activated.
 

10 hours ago, Dan62 said:

Try putting the GEM in service mode, the indicators and a tone should sound with each door being opened or closed.

l    Switch off the ignition
l    Switch off all other electrical consumers
l    Apply the handbrake
l    Shift to neutral
l    Close the doors.

Activate service mode
 
Proceed as follows to activate service mode:
 
l    PRESS and HOLD the switch of the heated rear window
l    SWITCH ON the ignition.
l    RELEASE the switch of the heated rear window

A signal sounds and the turn signal lamps illuminate to indicate that service mode has been successfully activated.

Note: Ensure the windscreen wiper is in the OFF position
If the alarm is activated (in vehicles fitted with an anti-theft alarm system), service mode cannot be activated.
 

This worked a treat! Rear passenger door (Nearside) is the culprit!