Thanks Liz, Glen and Scott.
Glen - interesting idea... I haven't shot them consistently enough for an animation to be worthwhile, but I did a quick-and-dirty alignment in Photoshop - have a look at this hack job
Scott - I don't mind at all, but it looks like you've removed real nebulosity and stars rather than the gradient! See the first pic - the gradient is in the bottom half of pic #2 as zodiacal light (bottom left) and airglow (bottom right).