Poacher
Gravitationally challenged member
Even given the Dunnock's unusual sexual behaviour (rampant polyandry and polygamy), they're normally totally shagged out by this time of year, so it's most unlikely to be a recently fledged youngster. Your guess of a drowned sparrow seems the best bet, especially given that there's what looks to me like a dry one in the background of your photo - they're gregarious birds, and I've often seen a bunch of them indulging in a communal bath while others seem to be keeping watch and waiting their turn.