One of my rear doors will neither lock nor unlock on the central locking. It's the same when pressing the remote key or the button on the dash. The door can be opened for them inside with the handle and can be locked f on the upside by pressing the knob down. All the other doors lock and unlock normally.

Is there anything I should check before ordering a new latch/ actuator unit?

BCU (body control unit) situated under the passenger seat. Easy to remove and replace yourself (1 hr). Bet u have slightly heavier power steering as well?

Steering feels fine.

What am I looking at with the BCU? Anything I can check with the IID Tool

interested in the answer to this as well - assume it is ok for mot if i make sure it is unlocked before they test it. Series 2a LWB Hardtop - ex British Aerospace that was then used in an armed robbery! not by me!
2 door V8 1979 Range Rover
A slight Subaru blip..
110 County 200tdi
R reg 300 tdi Discovery Aviemore - gone to a local enthusiast to be resurrected in Transylvania
2003 TD5 Discovery2 - now sold
and the 2 wheeled Range Rover - BMW R 1150GSA

and finally the big lady of my dreams... 2012 Westminster - gives me a smile every morning

the latch sounds like its ok...i'd check connections and power going to the actuator..if there is power then i could be the actuator..

Is the latch and actuator not just a single unit? I was looking at used ones on ebay (around £30) and they appear to be a single part.

yes it is a single unit, i meant the latch part of the unit is ok..