Thanks Peter :-)
I reprocessed this one again, hopefully for the last time (
see Astrobin). This time I used the same H/O data but blended it differently to create "synthetic RGB" stars. I had intended to just blend the colour from the "synthetic RGB stars" with the previous image, but couldn't get it to blend in a way I was happy with, so I reprocessed it. I'm not intending to get real RGB stars now.