As Greg suggested, would be interesting to see some calibration frames. Also some lights much shorter than 30 mins. Was that sub taken with a filter? What sort? Maybe your light pollution was bad enough to completely saturate the frame in 30 mins or perhaps there is a light leak somewhere? How are you converting the FITS file? Might be better to view the original FITS directly.
I wouldn't jump to any conclusions on the basis of a single 30 minute sub...
I thank you for your replies ad your thoughts and considerations with this matter...
I have been in lengthy contact with Bintel, who have been more than helpful. (Big thumbs up to Michael!)
I have been doing some experimentation and have to the conclusion that the image is a result of two factors...
1: The filter used - up until recently, I have only used my Ha filter. All imaging has been done from the backyard in a fairly light polluted area. The other night I changed to OIII. 30 minute exposures with a nearby fluro tube in the kitchen caused the bleeding...
2: Ghosting (residual image) of the CCD sensor - trapped electrons can remain for hours or days if it is cold enough. I was imaging at -15,so I think that explains the transfer of this to my darks later on.
Thankfully, it would appear that there is nothing wrong with the camera, just another issue due to the nature of CCD sensors!
So, once again folks thanks for your thoughts.