It took a bit of time to get the code as the wind kept blowing the telescope about, but eventually I managed to read it as 2H31 and then got a couple of dodgy digi-scoped record shots of the bird on my phone.
Checking the fantastic, www.cr-birding.org website, I reported the sighting to Paul Roper of North Thames Gull Group who got back to tell me that it had been caught and ringed as an adult at Pitsea Landfill Site in Essex in March this year.
This is the first sighting since.