I forgot I still had the front end part of the NVH kit installed!
This was blocking access to the central and outer gearbox mount bolts.
Soon removed with a bit of spannering.

this is it partially removed
Different angle
All gone and bolt head now visible

So the upshot seems to be if you have NVH installed, and you want to work on the rear gearbox mount, you will need to remove it ( temporarily or permanently).

Ironically the rubber on my old mount was in good condition and still attached to the mount bracket, but having got this far I changed it anyway for a new one.