[jase (Jason)]

Pondering on this for a while. I think its a mid tone management issue Clive. Actually, your unprocessed version is beginning to show signs of white clipping which is alarming so early on in the a routine. Perhaps this compounded your efforts as this would limit how far you could apply a non linear stretch (unless you were using layer masks to manage the intensity). What do you mean by 4 stops? Binning 4x4? I've attached a "nasty" rehash - not much I can do with a jpeg data, but I think it convey the point I'm trying to make around the fainter nebulosity - dark nebs top right and lower extensions (be it a very crude example).

EDIT: Now seeing your second image you uploaded above, I think you're on the right track. Undoubtedly, the intensity around the hour glass limited your ability to stretch the fainter regions. Masks maybe your only answer as you suggest.