Many thanks to all those who have commented so far. Still in the process of checking out all the angles but an encouraging response so far.
Keith Brockie posted an interesting comment earlier this morning:
Are there any photos showing the side of the neck? The easiest way to separate a summer plumaged arctica from pacifica is the extent of the white neck stripes. In arctica they extend right down the neck ‘flowing’ with the belly stripes. In pacifica the neck stripes end before the base of the black throat patch, look at any photos and it can be easily seen and to my knowledge has never been commented on!
The difference Keith describes seems obvious in some photos, but less so in some others for example this Pacific in Manitoba, by Martin Scott.
Sadly, I don’t have much in the way of decent, side-on images of the Grutness bird with the neck fully visible. This is probably the ‘best’…
Or maybe this one