QUOTE(speedypancake @ Feb 2 2006, 11:32 PM)

The only other thought is that my cable isn't an "official" Xbox but a 3rd party one. But it's got all the connections I want - RGB, component, s-video & optical audio.
Really don't know where to go next?? Any suggestions gratefully received

The question is: do the settings for HD appear in MS Dashboard?
If yes, enable some of them and reboot.
If no, your Xbox isn't seeing the HD-cable as an HD-cable, so it will not output HDTV no matter how effort you put into it. You can try modding the cable. However, I seriously advise you to simply buy the official one from MS (which you can get at Lik-Sang).
Again, this is what you will need to do:
1. Buy the official MS HD-pack from Lik-Sang.
2. Once shipped, use Enigmah Video Switcher to let your Xbox output NTSC.
3. Use the official MS HD-pack to connect your Xbox to your TV.
4. Start MS Dashboard. Since your Xbox is now outputting NTSC
and since a proper HD-cable is connected, the menu options will appear.
5. Configure MS Dashboard to at least output 480p.
6. Load up Xbox Media Center (or a game which supports 480p) and have fun.