Thanks for all the comments guys, especially alpal for the help
Graeme: Yes, after looking at it again this morning, I'm not happy with the colour balance. This has been a sticking point with a few of my images so far and some of it comes from using a light pollution filter. Once I get the background colour sorted, M83 is pretty much just blue. I had to add the red to the core myself. My camera is unmodded* so I'm not sure how to go about getting the Ha regions in the arms. Darker skies would always be nice but longer exposures saturate the stars. I might have to try shorter exposures and blend the stars through. I have to learn how to do this.
*I now have a modded 30d (thanks Troy) but haven't had a chance to test the camera properly yet.