Hi Graeme!
Welcome to the forum! Great first post, some nice detail there and the colour is great.
I'd suggest trying to stack less frames.. 60% of frames stacked is a lot! I'd go for 10 or 20%. The image can be a little bit noisier, but you just have to be careful about what other processing you do.
Can you explain your processing routine?
btw
to the forum! how did you hear about us? I notice you're from ASSA - there's been more and more adelaide guys signing up lately, great to see!