Caspian and Yellow-legged Gulls?

1st winter Caspian and Yellow-legged Gulls?, Milton, 29th December 2006, © Dick Newell

I'll stick my neck out on these two, as both could be challenged. With its gleaming white head, upright stance and longish neck, the 'Caspian Gull' looks good, except that its bill is a little short, the scapulars are rather heavily marked and the legs are not pale enough. Indeed in some respects, it looks more like a Yellow-legged Gull. So either it is an oddly plumaged Caspian Gull, or it is a strangely shaped Yellow-legged Gull or it has mixed ancestry.
The Yellow-legged Gull has a good head-shape, a mask behind the eye, good greater coverts and tertials, but the wings look a little short and there is no contrast between the scapulars and the wing-coverts. So is this a good michahellis, or, is it one of those challenging mich-like argentatus? The clincher is the single new inner greater covert, which eliminates Herring Gull (RGN).