I have now solved this with the help of Netgear support. The solution was to switch the DG834Gt from Modem/router to modem only (via
http://192.168.0.1/setup.cgi?next_file=mode.htm) then set up my ISP login details on the WNDR4500 and finally set up port forwarding. Having done that the content server is now visible externally.