This has been on our field pond for the past couple of weeks or so. Seems to be on its own. Today was the first time I'd seen it, and I managed to get a couple of brief zoomed clips from which the attached (poor quality) stills have been cropped out -- best I can do for now, I'm afraid. It dives underwater to feed. I haven't heard its sound yet, but my OH says it's a pretty loud "cackle", It's sharing the pond with a pair of Canada geese just returned for the 3rd year to breed, a pair of moorhens (which it doesn't seem to like very much), and a couple of lone male mallards. It's smaller than the moorhens.
I cannot find it in my (admittedly rather old) bird book, or (so far) in an online image search for a diving bird or duck with a rust-coloured neck.
ID help much appreciated -- thank you.
Richard B