Cheers for the comments guys. Also just realised there is another little smudger towards the bottom left.
IIRC I used DBE, Multiscale Linear Transform , might have used a wavelets as well can't remember.
Played around with other things just to see if there were any background improvements but I think the main problem might be unbalanced darks/bias/flats on top of losing orientation when I had to bring my gear in a couple of times.
It's a fairly heavy crop but one thing is that drizzling definitely improved the stack so it would have been a lot worse without drizzle.
I'm going to have another bash using the Pixinsight LRGB videos.
One question comes to mind, would you de-noise the RGB before adding the L or after?