Ok now we are getting somewhere your tips and advise has helped a great deal.
I have launched an instance and it provisions an IP from our external pool and that IP get assigned without any NAT, however when the VM boots the Raise network interfaces displays in the boot screen and it takes 5 minutes for the VM to boot.
I am sure this is not usual and would have thought that a static route would be needed somewhere as there is no external access.
OK the boot issue is gone by adding dhcp to the subnet.
Still now external access. ANy tips on routing that would need to be added?
Your initial command was mostly correct your just need the right order.
01. openstack router set --external-gateway <external-network> --disable-snat <router-id>
openstack router set --disable-snat --external-gateway <external-network> <router-id>