The mystery gull from the other day was of course a juvenile Herring Gull. It's interesting how similar juveniles of Herring Gull and Greater Black-backed can look and the variation in apparent bill size in Herring Gull is often confusing.
Here's a juvenile Geeb for your delectation.
Now, how good are you at estimating numbers? Take a quick look at the pic of the Oystercatcher roost and estimate how many there are - answer in a day or so.