Thanks Raymo, Comet, Kevin & Rusty
Yeah, I think I opened the throttle a bit on the noise reduction, I got to the point where I was reducing the noise that I was introducing (hence the mottling).
When I first started I struggled to see the objects, so I increased the ISO. Now I'm slowly winding this back again, and hoping to focus more on ISO 1600-800 as I get better at auto-guiding and longer images. I recently got some great advice over in the beginner's equipment section about how to improve my polar alignment & go-to's so this should also help