I saw this bird (which I presume is a 1st winter) this morning. It was in the company of Black-headed Gulls so a size comparison was difficult, but it did look bigger and bulkier than YLG or LBb. It also looked thicker-necked. The bill looks heavier than YLG or LBb with a more pronounced gonys, but didn't seem quite as thick and heavy as GBb Gull (which is a scarcity here, and I have only seen three or four in the past twelve years).
Any of you gull experts care to comment, please.