Thanks Rick. Had a play with the PixelMath blend method using some of the Deep Sky West data, following the instructions from Harry's videos and Jim's Cribsheet. I agree that it works a treat, and gives plenty of control. Next time!
I was pretty lazy here, using the RGB in tact from the 183mc and blending in some Ha using the "NBRGBCombination" script under utilities.
Cheers,
Jeff