AND THE WINNER IS:
Marc and Rick.
The problem turned out to be a screw up in image capturing. I blame upgrading my software but my Gain and Offset values for the camera were set at 14 and 133.
My dark library and bias frames had a gain and offset of 17 and 123 so the image calibration process was visually assaulting my frames quite badly. The crap bias frames would have introduced scaling issues.
I redid flats light night and then ran off 50 darks and 50 bias frames, and the result is below. Not sure where the gradient came from, but I don't care. DBE makes short work of that.
Funny story:
Actually I found the problem 2 days ago but when I ran the 50 darks and 50 bias frames I realised that I forgot to change the gain and offset values. I remembered 3 hours later, stopped the process, deleted the files and then noticed my telescope wasn't parked and was past the merridian, so I moved it parked it and then started taking darks and bias frames again.
Notice what I missed? 45minutes later I realised I still hadn't changed the gain and offset values.
. So repeated the process set the gain and offset and then got 1 frame out before I decided it was bed time. There's really no helping me anymore is there
Anyway thanks all.