Bar-headed Goose Anser indicus
Unfortunately this beautiful species does not occur as a wild bird in the UK, although a small number may breed ferally. A slightly larger feral population apparently exists on the Continent and it is believed that some birds from there turn up in the UK occasionally. However escapes from captivity are also encountered frequently and it will rarely be possible to confidently assign a bird to a feral Continental origin rather than a direct escape. The first photos below are of an individual which is assumed to be an escape, although a Continental origin cannot be ruled out.
The bird at Welney was feeding with Bewick's Swans which would have migrated through areas where Bar-headed Geese occur ferally so there is perhaps a chance it arrived with them; however it was reported at Welney on 4th October, by which date the first few Whoopers had arrived but not, as far as I know, any Bewick's.