Hi Graham,
Where the heck do you get all these clear skies from to do all this!? You're a machine!
Just responded to your PM re: gradient removal.
Wipe should definitely be able to do the trick (see example).
EDIT: to remove the gradient using ST, just launch Wipe and click 'Do'. Click the 'Fill mask' button if asked - it is often unnecessary to mask out objects and/or sample specific parts of the sky unless they obscure the real sky (buildings, mountains, trees, dust donuts, etc.). You can play with the Aggressiveness parameter if Wipe is being too gentle to your liking. I also bumped up the Wipe's noise/dead pixel filter somewhat as very noisy images and/or JPEG artefacts can 'scare' it into being too gentle. All this should remove the gradient effectively and you''ll be left with an even background. Note that there is a fair bit of chroma (color) noise present. Wipe does not remove this, but you can use the Chroma Noise Filter in the Development module. I further used ST 1.2's upcoming Wavelet Denoising algorithm and used the Isolate preset in the Life module (without a mask).
Rowland is giving some good advice with regards to Wavelets. Wavelets let you manipulate features of different sizes in your image, while leaving others alone. Indeed, they can be really useful to emphasize a galaxy's spiral arms. NGC300 in the JPEG posted is rather small, so it's a bit hard to apply it in there as an example.
Love your work!
Cheers,