ajohn

Could it have been a male Common Teal starting moult into breeding plumage?
Many of the male ducks are moulting at the moment and show unusual patterning. Teal often seem to moult the area around the bill early, so they appear similar to females but with chestnut patterning near the base of the bill.
This explanation should fit with the description of size, dark bill and blue-green speculum.