When I first started birding the estuary at Newport in the late 1990s, Common Gulls were decidedly not. They began to be counted for WeBS in 1994-5 & for the next 5 years the average of Oct to March counts was effectively zero. Numbers started to increase around the turn of the millennium & from then till 2012-3 the average WeBS monthly count was in the mid teens. I began sporadic counts in 07-08, presumably having noticed that at times there were much larger numbers.
Since 13-14 numbers have doubled or trebled, last Winter the WeBS average was 30, there are regularly between 50 & 100 birds, not infrequently >100. My max count is 150. The running WeBS average so far this year is 40 & today I counted 95. (Bird Track gets excited at counts of >50).
Annie kindly let me have the whole Pembs WeBS count for this period. They show a quite different trend; the numbers in the late 1990s dropping in the first few years of the 2000s but are now back to the 90s level.
Where do the birds come from?. The first ringed bird I saw was a Norwegian in 2012-3. This bird reappeared in 13-14. I had 1 sighting in 14-15, 4 in 15-16 & so far this Winter 5 more. Of the 11 individuals reported, 9 are from ringing schemes in S Norway & 2 from Hamburg, N Germany. Apart from being seen at Newport none of these birds has been reported away from the area of ringing.
None have been from any other scheme, though the cr-birding website lists lots of "Eurasian Mew Gull" ringing schemes from further E in the Baltic, Central Europe & N UK.