Light Pollution Filter
Well, I found out why the light pollution filter was giving my images a green / blue bias.
The filter chops out pretty much all the yellow / orange spectrum. Which is not good, as that also chops out a lot of the visible star colours.
So what do I do?
Do I remove the filter and swear at all the light pollution (and faff around in post-processing to try and get rid of it) or do I leave it in and have images with a disappointing blue / green cast?
Makes me wish I had done more research before ordering the thing, now.