Can anyone help ID this bird? It was surprisingly large (bigger than a mallard) and seen in a park in south-east London. Initially thought wigeon or pochard but the breast colour doesn't seem to be right for either of those.
Not a duck but a small species of goose, Egyptian goose. They're not native to the UK, but have been introduced and feral birds are not uncommon.
The head can appear darker in contrast to the breast in sub-adult birds; see the gallery at the Wikipedia link above.
They're not always in books as (apart from very rare vagrants) they're feral descendents of introduced birds