Hi John,
It is the presence of several features that point strongly towards Glauc: that it is a huge brute, easily Greater Black-back size (tho' perhaps not quite so long-winged); the bill pattern with a sharply demarcated solid black tip to an otherwise pink bill (which it also had last summer, when still just a first-summer bird: Herring wouldn't show this), the head shape, etc.
When it was first found (6th July last year), it was actually identified as Glauc at first, it was only later (late September, tho' it had been missing for a while) that people realised it also had Herring features, particularly after it moulted and got fresh(er!) plumage showing the dark wing & tail tips (it was very faded & worn last summer!) and the heavier mottling on the breast
Assuming Edward is right that it's from the hybrid population in Iceland, there's no reason it should be a first-generation hybrid, it could well be an nth-generation hybrid, which could show just about any combination of GG and HG features, not necessarily exactly half-way between as a first-generation hybrid would tend to be (Edward, is this true of your gulls? any thoughts on where you'd fit this on a scale GG-HG?)
Michael