In late Summer it's not unusual to see Sandwich Terns with the gull flock. There was 1 today, & a mini flock of 6 about a week ago.
Arctic & Common Terns are much more unusual, I don't see one every year, & then just 1 or 2, usually on the outer estuary.
So a flock of 24 Common Terns (c1/3 juvs) with the gulls this afternoon was pretty unprecedented.