Well done Jason, you were right - they were indeed blue-winged teal. The blue really is spectacular in flight. These are rather flighty here so unfortunately I see them in flight more than I'd like (as they generally take everything else to the far side of the pond with them as soon as I arrive).
Eventually a couple of them came back round and landed. The drake is above, the duck below.
Very smart birds these; both in flight and swimming.