My guess is that it's part of a fan-built Dalek. I find it hard to believe that a Dalek used for filming on the show could be tossed in a pond like that without the story circulating among those mega-fans who know the details behind the shooting of every episode.
Of course, my mind still reels at the utter inanity of the BBC's old program-junking policy. I suppose chucking away a broken Dalek head wouldn't be any different than that.