I have attached my best efforts on this and thought I would just set out what I did in case it helps anyone else.
1. Use StarNet process to get starless image and a star mask. This does not remove Sadr!
2. Use RangeSelection process to create a mask that gave a doughnut round Sadr protecting the star itself, selecting the area around the star where the diffraction pattern is and falling off again after that (smoothness setting of 18).
3. Use the CloneStamp process to blacken the rest of the range selection mask so the only operative area was around Sadr.
4. Apply the range selection mask to the starless image.
5. Use MultiscaleLinearTransform on the masked starless image to smooth out the area to reduce the diffraction pattern - lots of to-ing and fro-ing here, working on a preview to try to balance removing the diffraction pattern while not smoothing the rest of the area more than necessary.
6. The area now looked a bit over smooth so I used the NoiseGenerator process to introduce a bit of noise - again lots of tweaking to get an appropriate/subtle level.
7. Finally use PixelMath to replace the stars.