Thanks for the comments all. I have given the data a bit of a more severe beating with GradientXterminator.
It is in the same place
http://members.optusnet.com.au/~trlee8/VelaSNRmos.jpg
The problem with 'eliminating' gradients is you start to lose contrast of the faint real nebular data. Most of the nebs in this image are very faint.
Bert