Wow, this is a sickened but also quite exciting. If it is a Slaty-backed I would love to go and see it!!!
When I first saw the picture it looked just like photos I have looked at of Slay-backed Gulls. Top gull watchers can't really seem to see any problem with the bird.
Here is a description and full write up including photos: http://www.birdingetc.com/
Until next time, Foghorn out!