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Very definitely any light pollution makes flat fielding quite a bit more difficult.
Both my sites are relatively dark, my dark site is dark dark and my home site has semi rural skies so relatively dark especially to the west. It seems dark until I go to my dark site and then the difference is obvious. A real dark site makes the stars look fiery.
You can try sky flats. I think you simply take more flats with no cover on the scope and then median combine to get rid of the stars. Make sure you dither a lot between individual flats taken. Now that's all the wavelengths you are imaging in.
Greg.
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