I did and it appears in uncompressed TIFF too. It does't happen often but it is definitely associated with the Nik processing.
Ok I did wonder it it was causing the banding which is accentuated by processing.Thanks, Julian. No, the sun was quite low and the sky was a nice blue to start with and was enhances a bit (but not that much) in post. Polarisers are a fiddle with a rangefinder.