I was then away and out of touch for a few days, so the next chance I had to see it was the Friday. When I went to Donmouth on Friday, I saw it in different light and immediately had concerns about calling it a Thayer's.
I spent most of the Saturday with the bird and these views further cemented my view that it was not safe as a Thayer's.
Mine is just an opinion, based on where I have set my line between a safe and an unsafe Thayer's. Some details of the primaries (darkness, pattern at tips), the secondaries (overall darkness) and tail (e.g. extensive marbling along tip) make me uncomfortable calling it a Thayer's here in NE Scotland.
Here are some images.... All are in dull, neutral light, with absolutely no changes to images other than cropping.
and for comparison, shown below is one of my images... not looking so good for Thayer's in this different light/with this different camera/with this different background.
My sincere thanks to the many top birders who have been willing to share their views on, and insights into, this great bird.