Wow, was it showing well, right in front of the hide, as these images demonstrate!
The 'cold' grey and white plumage certainly fits with an 'eastern' origin but in the grass, difficult to assess the hindclaw.....
Eventually, it hopped onto the mud and the long hindclaw is clearly visible - an indicator but not conclusive for an 'eastern' origin.
The long hindclaw.....
And a final portrait of this cracking bird :-)
The key to it's identification is of course a recording of the call. I managed to get a flight call on my phone but it's too big a file to upload. It sounds identical to other recordings I've listened to on the net. I know others have got recordings so it will be wait and see? Call will indicate it's within the broad 'eastern yellow wagtail group', maybe a feather will become available and narrow things further to a subspecies group?
The day ended on another high note with late evening views of the Pallas's Warbler on the Garrison, no images managed - yet another Scilly tick!