Thanks Rowland, Tony.
I took the imaging train apart yesterday and gave it a very careful clean. Google is my friend here
I didn't see any dust or marks on the sensor, but gave it a clean using a very soft brush and a blower thing. I've got some special liquid, lens cleaner, polypropylene & ethanol, as well as UHTC coating cleaner from Bintel. Should I use that also?
On the dithering, I tried playing around with it in MaximDL but don't think I've got the hang of it, I end up with pictures that have bizarre black pixels mixed with very light pixels around the stars. Google isn't my friend here as a quick search tells me I need more theory understanding before practical results improve.
So...Understand what dithering is meant to do thanks to this conversation, but don't yet understand how to implement it correctly.