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I think its just worn Chris, dark scaling is eroded away. I think your ID is correct, havent heard of any other moths that look so close to small magpie but are other species.
Chris Brooks - www.dragonfly-images.co.uk
My Flickr site - www.flickr.com/photos/ceb1298
Lat/Lng: 51.2, -1.1
OS grid ref: SU6547