Thanks for the posts and help.
I've actually realised that I had solved this unintentionally, but the advice above will come in handy if any future updates / patches break it, so I've noted that down Thoumsin, thanks.
BEFORE I got Entrenchment, 1680 x 1050 was offered as a resolution on the video menu. I set it as that when I was playing the original Sins, and it worked no problems. However, once I downloaded Entrenchment, and ran it, the 1680 x 1050 option wasn't on Entrenchments video resolution list, and it was displaying in 1024 x 768 (looked really stretched out on my monitor).
Whats weird is that when I launched the original Sins (I was gonna play the original until someone came up with a way to fix my resolution issue), the "Get Entrenchment" link that I had had changed to "Play Entrenchment". I figured I'd check the video resolution options on the original version to make sure I had it set right. The strange thing was that even in the original version, 1680 x 1050 was no longer an option on the resolution, yet the screen was displaying in 1680 x 1050 (smooth textures / proper sized text / clear images etc). I pressed "Play Entrenchment" and it re-loaded the game, let me into Entrenchment, and carried my settings over, so Entrenchment is now playing in 1680 x 1050.
Really weird I know, and I've no explaination for it, but hey, least it seems to be working for now!
Thanks for the swift responses. And yeah, 1680 x 1050 isn't so big - Monitor is only 22 inches. 
