Ok, I was just getting this ready to post, then saw Martin's unreal Vela SNR. Nearly didn't bother posting, but will anyway to show how persistent I am
I think I'm getting there with the blending of channels/filters. This is straight SII in red, Ha in green, and OIII in blue, but I found I had to boost the SII and OIII by 7.5 times the Ha to get the nebulosity to start showing some blues and yellows instead of all green. I just did this with PixelMaths. No playing with channels or hues afterwards. Obviously, because of boosting the R and B channels so much, the stars go that magenta colour and it's very apparent why they're always these colours in the Hubble images. I might go back and try to figure out how to blend in stars a bit more natural coloured with PixInsight.
Probably not the best image scale for this target, but it was in the right part of the sky at the right time.
Comments/critiques/advice very welcome please.