So, my first attempt at processing with PixInsight, I think that this it's a slightly better result than my previous attempt, although PI is pretty daunting and complex to get to grips with. And I'm not sure whether it's "better" or just different...

I actually used DeepSkyStacker to register and stack my frames. Then I split the image into it's RGB components using PI and recombined them using PI's Colour Calibration tool. I also fixed the background a little using the Dynamic Background tools.

Finally I did the usual curves & colour balance in Photoshop.