Thanks guys and gals for the comments.
I'm aware of the red. You should have seen the unprocessed version just after stacking. I wasn't sure how to handle it, because I know there's a lot of nebulosity in this area. I set the black points at the darkest areas - top right corner and also that dark patch at 9 o'clock middle.
Thing that's really bugging me is the kind of magenta tinge. Is that the real colour? I thought it should be more red. Couldn't for the life of me get the colour balance right the way I envisaged it.
@Brendan - if you're using Photoshop, use the info window instead of eyes. Press F8 to bring it up. Hover mouse over the area you think might be black/neutral, and you can see the R, G, and B values at that point. You can set it to use a 3x3 average instead of a point, smooths out noise etc. Very handy to see colour biases using numbers.
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