A Slaty-backed or Vega?

I found this one at Spa Beach last week along with many other Vega/Taimyr Gulls and the smaller Black-headed Gulls. The bird strikes me with its Slaty-backed -like figures but the mantle doesn't look as dark as normal Slaty-backed. The brownish marking on its head and neck looks much softer than Vega and is not clearly spotted like Taimyr (2 individuals in the background), and the legs are also richer pink. Its head structure also tends to look like a Slaty-nacked rather than Vega or Taimyr. It is distinctly bulkier with shorter primary-projection. Can this possibly be a hybrid? But from what I've read from Nial Moores' identification note in Birds Korea webpage (which is here http://www.birdskorea.org/Birds/Identification/ID_Notes/BK-ID-Slaty-backed-Gull.shtml), there is yet to be any solid prove of Slaty-backed Gull hybrids. Actually the gull in this entry also seems like a hybrid (Slaty-backed x Mongolian?) in my opinion. These birds are just too strange to be some normal variations.

It really has a body structure of a Slaty-backed Gull.

Comparing to the normal Vega Gulls; note head streaking and head/bill structure

Though its mantle is a bit darker than usual Vega but nowhere near the normal Slaty-backed.

A Vega Gull showing normal primary molt of this time of year; all new feathers, p9-10 still growing

And a normal Slaty-backed Gull from several weeks ago