Hi Jim,
Another vote for Arctic Warbler, and I agree that it has to be a different bird to that reported prior to your sighting. The strong greater covert bar (not seen on the initial bird) and very obvious supercilium exclude the commoner Phylloscopus warblers, and, no doubt, you can see why it's not a Yellow-browed or Pallas's, so that leaves Arctic and Greenish. The latter would tend to show broader and more diffuse supercilia in front of the eye, often meeting on the forehead, greyer legs, shorter primary projection, look more compact in general and so on, as distinct from the supercilium narrowing in front of the eye, long primary projection, pale legs and so on on your bird. Two-barred Greenish Warbler, though an extremely rare vagrant to Britain, can be excluded on structure and also, on Two-barred Greenish, the greater covert bar extends to the innermost coverts (inner two or so not prominently tipped whitish on Arctic), there would be more of a median covert wingbar (though some Arctics, especially in autumn, show this), and the legs would tend to be greyer (lead coloured).
Regards,
Harry