Thanks for the reply Binastro,
Yes it's a tricky area to photograph.
If you look just above the left arrow you'll see a dark ring which has an edge similar to a map, suggesting that whatever is inside stops at that point.
I wonder if Nikon or a camera specialist would service / clean it ? It doesn't seem to be affecting anything but I just don't like the idea of it.
Do you have any idea if these nitrogen filled optics can be refilled ?