Just for the record, Hannu Koskinen's thoughts(forwarded by Alan Dalton who told me that Derek Charles (Alan's birding mate)had been in contact with HK regarding this gull. Hope Hannu doesn´t mind.
Hannu's thoughts on the bird below for interest;
I think it is a good example of mystery Gull from Russia. Not a cach (too dark grey, prims wrong, primary moult too slow etc.).
Here start the guessing part: i think this bird might originate from Ivanovo district. An area where argentatus, cachinnans, heuglini and in lesser numbers barabensis and fuscus breed in same areas. Hybrids occur. In my opinion the moult score of this mystery bird fits better to pale LBBG (in this case heuglini/barabensis). Heu x cac possibly. Very hard text and to say this, because clear evidence about this kinf of hybridsizing is almost totally lacking in the litterature. Greg Neubauer et al have ringed birds there now in 2 summers, but all we know is that strange Gulls do strange things there. One of those ringed juvs last summer did the journey to Catalonia, Spain (like you remember the discussion in Scotland?). Huge range, never possible to any argentatus, but possible for a some kind of (pale) LBBG form. or hybrid.