@Thomas, now that you’ve mentioned that, I’ve looked back at my images, and noticed the one I uploaded was the -2ev exposure from a bracketed set of exposures.
@afre, i’d be interested in seeing the steps you took to show those circular artefacts. I’ve done some googling, these artefacts may be what is known as “Newton’s Rings”. Perhaps caused by the fact that I had a UV filter, and the 10 stop nd filter on the lens? The reason I put the UV filter on, was to save the lens filter thread from screwing on and off the circular nd filter.
Must save up some cash for a square filter system 
Lots of lovely edits of my raw file, so I’ll use these as inspiration for a better edit of my own image (this time using the properly exposed image, rather than the -2ev bracket image)