A couple of quick Qs Leon.
1. Do you adjust your focus, camera position or anything between taking your lights and taking your flats?
2. What is the spread of the histogram of your flats? ie all down the left hand end, in about the middle spread narrowly but evenly, all up the right hand end.
3. Have you tried not doing the DD and taking the image into PS and doing an AV (anti-vingetting) mask?
(
I just re-read what I wrote.
I mean anti-vignetting, not anti-annabelling
)