Exciting news, well for me at least. Josh and I located a yellow-ringed juvenile Yellow-legged Gull 'H8J0' at the stone barges at Rainham on Saturday (21st July). A bit of detective work revealed it was part of the Helgoland ringing scheme in Germany - thanks to Paul Roper and Luka Jurinovic for this.
The bird had been ringed in a colony in central Frankfurt, as a chick, on 18th May 2012 - it was also seen by Dave Walker at Dungeness, Kent six days before our sighting on 15th July 2012. I've always wanted to know where all these YLGs that accumulate in the Thames come from - the southeast locality would suggest they're coming across from the near continent as opposed to Iberia.