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.
Code
iptables -A INPUT -m conntrack --ctstate ESTABLISHED -i tun0 -j ACCEPT
iptables -A INPUT -p tcp --dport 51413 -i tun0 -j ACCEPT
iptables -A INPUT -p udp --dport 51413 -i tun0 -j ACCEPT
iptables -A INPUT -i tun0 -j REJECT
iptables -A OUTPUT -d 192.168.0.0/24 -p tcp --sport 9091 -m owner --gid-owner debian-transmission -o eth0 -j ACCEPT
iptables -A OUTPUT -d 192.168.0.0/24 -p udp --sport 9091 -m owner --gid-owner debian-transmission -o eth0 -j ACCEPT
iptables -A OUTPUT -m owner --gid-owner debian-transmission -o tun0 -j ACCEPT
iptables -A OUTPUT -m owner --gid-owner debian-transmission -o lo -j ACCEPT
iptables -A OUTPUT -m owner --gid-owner debian-transmission -j REJECT
iptables -A OUTPUT -s 192.168.0.0/24 -o tun0 -j REJECT
Is that all I need?
Thanks for your help.