Many thanks to everyone for their comments and tips, much appreciated
it's certainly been a good learning experience.
I've taken another look at the data and rescued another hour of data, not at the sharp end of the subs but at least/maybe contributing a bit more colour. Going through the repro, I hope I somewhat neutralised the gradient and I was able to tease out the Ha regions a little more than before. There's a bit of a blue cast to the background that seems to disappear if I clip the black end a bit, but I don't want to do that.
For now, at least, this is my final go
it's a little more colourful, a little more detailed and the stars are a little sharper, even though it's probably impossible to tell with the file size limit and compression
Cheers,
Dunk