any idea which bird may have owned this feather
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There are no feathers on a waxwing that would show this pattern - I can see why the suggestion has been made though, it's hard to come up with many suggestions for this feather.
More details, including the size of the feather, and the date and circumstances on which it was found would be useful, but I suspect that it is a fairly large feather (too large for most 'garden birds').
My suggestion is that it is a feather from the upperparts of a breeding plumaged Great Northern (or possibly Black-throated) Diver.
Compare with this image from a study of Common Looks (the American name for Great Northern Diver):
Lat/Lng: 54.9716, -2.1025
OS grid ref: NY935641