This is a DNS problem, not WordPress. ping test.onlinebanter.co.uk displays unknown host. WordPress does not even handle the requests because the client is unable to determine the IP address of the server and therefore does not send the HTTP request.
ping www.onlinebanter.co.uk works, so I assume the name server did not properly store the DNS configuration (yet). Depending on the setup, your provider might need some time to apply the new configuration to the name server. Make sure that you activated (or however it’s called in your configuration interface) the changes.