I guess this is both 'Advanced ID' and 'Rare Birds in Britain'
Photos and bit more info here
Please note the obs (top guys) are acutely aware of the enormity of the record and wish they had better views and photos but felt it was times to put the word out. All they have seen in the field and that which is discernible in the pics point to a correct ID in the tricky circumstances (of a tricky ID). Have a look.
PS I have also update the blog to include a little note and illustration on separating 1st summer White-winged and Stejneger's (having already been questioned on it).