Thanks for the tip - and now it looks like I'll be going full circle - I was doing Gain 75 the other night, but for far too long according to the tables.
I just had Pixinsight crash as I was trying to make a rough assembled image to post. Fortunately I've got the base data saved so I can have another go soon. I wasn't too impressed with the results I was seeing though - it seemed very noisy.
Edit: no, something has gone wrong somewhere in the process as I'm getting banding (I think that's from the master dark which I've reprocessed), but weirdly I'm getting a deeper black near perfect circle area that's not apparent in any subs I can see. If I can't work it out, I'll post it elsewhere to troubleshoot.