I re-did the blue channel in 3-minute subs, getting 27 of them before the target sank below my hedge. This larger number of subs reduced the noise in the channel quite a lot (although it is still a bit noisy).
Also reprocessed more carefully, especially by taking the individual colour channels into Photoshop and removing the skyglow gradient from each (using Gradient Xterminator).
This is starting to look good; I'm pleased with the direction. If there are more chances this season, I'll add more subs in all the channels.