Driver's rear door won't open. Other doors are fine. Won't open via either handle. Child lock is off. Tried all unlocking ways using keyfob. Can hear central locking working. Cannot open the door. 

Any ideas how to open the door?

Sounds like the latch mechanism is broken.

May have to remove the door panel from inside the car and release the latch manually.
It can be done and if careful enough without damage (that can be seen)

I've got as much of the panel away as I can....so much that I can slide my hand down one side and click the child lock on and off. I can't see where I can get my hand down anywhere else as the latch is the other side of the solenoid and is mounted. Are you sure (no offence intended) that the latch can be undone?

No, you probably wont be able to remove the latch mechanism but you could try pulling the lever up/down to release the latch.
It may be that the central locking servo is not operating the lever so try locking and unlocking to see what is occurring.

Worst case scenario is to destroy the plastic on the latch mechanism and force the latch into the release position with a bar but obviously that would necessitate replacement which if faulty then nothing lost

Lever?

How can I get to the latch to force it?

Where the cable or rod goes into the latch mechanism is where you need to be looking at to see if there is anything you can do to influence releasing the door.
I have in the past used a long lever bar and hammer to break the plastic housing around the latch area.

I have a long thin flat screwdriver and a hammer. I have the replacement part so I'll take a look at it and see where I need to aim the screwdriver to influence the latch. I'll try it 2morow and let you know. As soon as I get the door open i can install the replacement.