thanks all for the feedback
Ken - I should have been more specific in that I have replaced the stock canon UVIR filter with an Astrodon Inside Luminance filter (see
here).
When I first modded the camera I kept the anti alias filter in place, however I have subsequently decided that this was contributing to excessive FWHM values, and so when I had the camera disassembled on the weekend I decided to remove that filter as UVIR filtering was provided by the astrodon.
RIck - I agree the stars are still not perfect, but I am thinking (hoping?) that this is now related to an overly tight clamp on my secondary mirror rather than with the camera (especially since I did not see this when using this camera on my VC200L). Am planning some further investigation over the next few days.