On Sunday afternoon I was walking round the large lake with Zoe when we saw this pair of pale brown, almost white ducks. They seemed a bit smaller than the mallards that they were with, but this photo by Dave Appleton (halfway down this page at gobirding.eu) of a very similar bird in the Scilly Isles, suggests that they are probably a domestic breed of mallard (Anas platyrhynchos).
From the relative shapes of the heads I suspect that the one above is a male and the one below a female.
Photos taken in Whiteknights Park, Reading University grounds, Reading, UK, on 2010-01-03.
Zoe tells me that these are call ducks which are, just as I suspected, a small domestic breed of mallard. Since about a week ago, one of them seems to have disappeared and the remaining one has been hanging around with a male mallard.