Funny what you can find on images. About 1/3 of the way from the centre of NGC 4945 to the edge of frame towards 11 o'clock there is a small circular fuzz. The maps in Skytools (go down to mag 20) show a star there, so I thought it was some processing artefact. But it's on all the raws as well. Maybe I'd nabbed a faint comet! But it's visible on the DSS images of the area (Skytools is great as it can just download them and put them on the map). So not a comet
I could not find it anywhere until I found this obscure reference:
http://adsabs.harvard.edu/full/1964ApJ...139..899D
It just says it may be a dwarf companion. I thought it was a distant background galaxy. I can't find anymore information about it, but would love to know more about it if anyone knows how to find out more.