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Thanks everyone for the suggestions.
The images were taken using the iTelescope service in New Mexico. Images can be downloaded pre-calibrated or you can download the calibration frames and calibrate them yourself. I'm pretty lazy so I usually use the calibrated images.
Looking at the master flat more closely, it looks like the flats have not been combined properly or dithered. A screen shot shows the problem - looks like a single sky flat with the stars retained. Alternatively it could be hot pixels.
Anyway, the Cosmetic Correction tool is what I was looking for. I managed to remove most of the dodgy pixels and now have a reasonably clean image. A screen shot of the cleaned up affected area is attached.
I remain a PI novice - but each image encourages me to keep on learning. With this one its been both the CC tool and Star Halo Reducer script. I'll post the full image in the Deep Sky forum.
Thanks again
Pete
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