White-winged gull conundrum

Taking a break from work this afternoon, I headed off to Cattle Point to see what I could find.
Offshore, there were good numbers of pigeon guillemot plus a handful of rhinoceros auklets. A few surf scoters and harlequins were kicking around.
A small flock of a dozen or so spectacular long-tailed duck flew in and landed on the water.
7 brant were flying around offshore.
Searching the shoreline for waders I could only find a group of 13 black turnstone, and at one point I heard a greater yellowlegs but failed to locate the bird itself.
Pelagic cormorants were perched up, posing nicely - hence the accompanying pic.
The only passerine of note was a savannah sparrow, seen feeding along the tide rack.

Among the muddle of gulls by the slipway, I spotted what I initially thought might be a Kumlien's gull but on closer inspection, I wasn't convinced.
Some of its plumage features suggest that species, but its small size, head shape and dainty bill had me leaning toward a leucistic mew gull...?

Please have a look at these photos and see what you think. It's certainly a curious looking individual.
I'd be interested to see what others think.