From your screenshot - that is exactly how the demo looked on my tv. The full game for some reason had basically the centre bottom half of the screen - half the minimap is cut off, as is the A and Down hotkey icons. Also if the 4th dialogue option has 2 lines then the bottom line is cut off. It's like it's zoomed on the lower centre section of screen.
I spoke to the guys at the game store, who said because it's christmas I will have until mid Jan to return stuff, as opposed to the normal 7 days. So I'm going to hang onto it and try a few of your suggestions [I tried most of them last night but I'm gonna see if there's a problem with putting it on a relatives Xbox while I'm off on vacation for the next couple of days].
FWIW - I'm using the HDMI cable with my xbox, and I run it at 720p at 60hz normally. I will try connecting my xbox to my computer monitor - I hadn't before because 40" > 17", so it seemed a little pointless.
Thanks for your advice so far, here's hoping.