Back in August 2011 i was birding on my local patch i was taking photo's ,of tufted ducks when two ducks landed one in the middle of the lake one about ten feet from the bank i did take some pictues. but lost my sd card ,but the sd card has now turned up after going through the card i wasn't sure at the time what duck it was but after doing some research The birds are Ringed Teals they come from South America they were probably escpees i think but i don't know .
A few years ago I encountered the same species at Bouch Beech Reservoir in E Sussex. That one was an escape, and I would presume that the birds you saw were too. They are kept at London WWT, for example. Hope that helps- interesting record all the same. (M Bournat)