Very nice Andy. As Rick noted this is a green gradient (light pollution probably). HLVG free plug in for Photoshop gets rid of that in seconds.
http://www.deepskycolors.com/tools.html
SCNR in Pixinsight does the same.
It will balance out the colours nicely. Green sometimes comes from airglow as well so you can still get excess green sometimes even if you image from a dark site.
Greg.