Well things are sort of getting better. I was using the bridge-up.sh script from here...
https://docs.openvpn.net/how-to-tutorialsguides/virtual-platforms/site-to-site-layer-2-bridging-using-openvpn-access-server/
... but have no swapped over to the one from this URL...
http://openvpn.net/index.php/open-source/documentation/miscellaneous/ethernet-bridging.html
... called bridge-start and the bridge interface on the client now appears to be working. I have a switch plugged into the south / private interface of my bridge client and the ports are blinking like crazy and a laptop plugged into it has been given a DHCP address.
However it appears that traffic is not flowing correctly over it though, in that I cannot ping anything at the main / parent site or connect to any resources there. I wonder what I'm now missing or not doing ?.