Thanks Allan, Rick, Greg and Mike for the feedback. I've taken it all on board and have had a final tweak to a) Up the red balance slightly to better highlight the Ha regions and b) Added some brightness to the high end of the histogram to make the galaxy pop a bit more. It's been a useful exercise to hone some processing skill.
Large Image updated in-situ at same link:
http://www.pbase.com/david_fitz_henry/image/154981740
Also attached the latest small image below.