Learnt a lot and finally have OMV, OpenVPN and Transmission working correctly together on a Raspberry Pi2.
The main requirement was to block Transmission if the VPN disconnected.
Just hoping someone would be so kind as to check my Firewall settings. I am using the OMV Firewall gui but I've shown them here as iptables for ease of display.
Is that all I need?
Thanks for your help.