Steve,
Some years ago a friend and I found a Phyllosc warbler at Radipole (Dorset), with huge bill, prominent long supercilium, two wing bars etc. We took notes, Bear did a sketch, and then and only then did we sit down (over Spaghetti Carbonara and Chianti, for Peter's interest !) with the field guide to try to ID it. We were left with the choice of Yellow-browed Warbler or Arctic Warbler. We both independently wrote down what we thought it was, had a sweet (profiteroles) and coffee, checked it all out again (with the help of the Italian café owner and his wife, who'd somehow got involved), and all agreed - Arctic Warbler.
We submitted all our evidence to the BBRC, and it took them nearly a year to decide we hadn't completely ruled out Yellow-browed Warbler. So that was that - but every time I see a photo in any of the bird mags of an Arctic Warbler, I'm still convinced we were right.
Tony