Hi Theo,
Yes, you could very well be correct about the frosty bird being fuscus. Both jizz and plumage look good, and of course the jizz is probably better for this taxon than heuglini. One problem was establishing what might be passing through Azerbaijan in Sept (virtually nothing known about gulls there). I imagine that fuscus should pass through regularly en-route to the Middle-east. Based on ads, heuglini was rather abundant, whereas I only saw 1 adult fuscus.
DSLR photography there was difficult. I spent around 8 days at one particular site where there was a flock of 2000 or so gulls. It was on a beach but they were difficult to approach, even in my car. I spent many frustrating hours trying to approach the flock. Only on one afternoon (and even then only for a couple of hours) did I manage to get close enough for reasonable pics, and so over the short time that i was near to the flock I rattled off as many pics as I could. I was keen to get shots of as many birds as Icould and so had little time to follow individuals to get pics of underwing, upperwing and tails, especially as the birds very very nervous and spooked easily. So I just moved through the flock getting pics of standing individuals. This is my (not very good) excuse for only have these standing pics of these non-Caspian types
Its a brilliant place for gulls and no problem to watch them in a telescope or take digiscope photos, but tricky for me and my 400 mm lens.
(As an aside, the reason every single casp is facing right in the pictures is that they were all on same afternoon, facing into wind and I took pics of each bird in turn as I drove along behind the flock!)
Chris