Thanks all for the kind words - and yes, noise was the key issue. The gradients are mainly due to stray light in my neighbourhood that I can't quite remove (though maybe with more skill I could!), but the noise was an unavoidable consequence of pretty hot, hazy nights - the overnight min temps were 18.1C and 22.7C, so at late evening imaging time, likely very well over 20C for both. Not the best for uncooled DSLR work! To be honest I'm surprised I could get anything much out of them
There were darks and flats removed for both.
I'm gradually learning a fair bit more about PI's noise reduction tools, and so I'm not going to explore into other NR techniques now, despite the kind offer Brent (
). It's such a learning curve, that I feel I should get on top of one (or at least reach the limit of my skill) before trying another method, at least for now.