The Openvpn server is on an Ubuntu box so I am not sure I want to start changing that around for a 1 off pain in the rear server that is so old and obsolete that the company that supply it dont really support it!
I will try the snat route and see if it works. Luckily all the kit is currently on the bench so if I break it its easy to fix, but it will be installed on site in a couple of days.
Thanks for your help.