Thanks also DJT and skysurfer - for some reason I can't quote either of your comments!
THanks also to Colin and Rick who provided some pixinisght insight. I've experimented a bit today with the local normalisation and have concluded that it doesn't add much when you have very blotchy and changing background gradients. Using TGV more aggressively and then the MMT to fill in the black holes using a luminance mask seemed to improve the background a lot. smoother and more clipped background version here!
http://www.astrobin.com/314169/B/