The video below is great for seeing them just as I did. The poor light made it difficult to re-produce the grey on the mantle and scapulars as I saw it through my scope, but the first one on the bank shows how pale silvery grey they are. The first one is preening its breast in front of an out of focus piece of vegetation.
The last bird shows more contrast in the plumage than the other two and its dark bill adds to this effect. Check out the white underwings as it takes off. Other large gulls species will have more brown on the underwings at this age.
The first two are typical of 2nd CY Caspian Gulls at this time of year, but the third one appeared more advanced except for the all dark bill.
Head size appears to be variable amongst Caspian Gulls with some birds having noticeably small heads whereas some not so. Studying photographs, it may be that the head size decreases the further east they come from. Chris Gibbin's photographs of Caspian Gulls from the Ukraine seem show them having larger heads than those from Azerbaijan - see the list of blogs that I follow for a link to Chris's blog. I have seen some really small headed Casps with the one in the video HERE having a small head. It is impossible to know where most of our birds are coming from, but there have been some colour-ringed Polish birds in the county. Unfortunately there are supposed to be quite a lot of hybrids in Poland!
As always feel free to offer any comments or disagreements: