Hi Chris, thanks for the info and efforts!
You're probably right about pushing the black a bit more - bit it's a tricky balance between getting too much of a 'posterising' effect and not having a very rich black. More experimentation required!
Yeah, I think you've gone a bit far with the adjustment, but not easy using the jpeg as opposed to the RAW or TIFF files.
I ran a median filter to smooth the noise on the single (2nd) image and tweaked the levels to darken the black for another version, still need to work on the stacking though!