For some reason I never got round to sending the details of the bird off until this week. I emailed the BTO the details of the colour ring (left leg, white with black writing P2R7) and was very surprised to get a reply the following day. The email I received was from someone in Estonia telling me that the bird I'd seen was an adult female Common Gull, that had been rung on 29th May 2010 as a breeding adult in a reserve called Matsalu National Park in Estonia- result!
As well as my sighting, the bird has been reported another three times; twice last year in Essex (August and December) and once again this year, back at it's breeding site (May 2011). This is great news, as it shows that it survived last winter's awful weather and made the return journey to breed again.
Getting this information really makes me want to go back to Essex to see if she's back again!