It's what's on the white ring that will tell you about the bird Richard, I can't make it out in this shot though.
Sometimes they will feed for a few days and then head home, others just end up joing a local flock of ferals. If you catch it it is possible to contact the owner and arrange a return.
Note from Rosemary Royle: We tried that once and he said "Don't want it back - it's obviously useless so wring its neck." Which of course we didn't. I hope it lived a long and happy life with the feral flock at a local farm!