Bi-colored bill with pink based suggests Glaucous Gull as one parent. Dark primaries point to Herring as other parent. Checkery greater coverts should exclude pure HERG or WxGW hybrid.
Seen at some distance flying over the Cromarty Firth. I was in a small boat in the firth at the time of observation. Apologies for the poor shot, but i am curious to get some insight on the large wingspan bird.