Oranges and Lemons

"" was a nursery rhyme. It was Charlotte Pollard's favourite when she was a child. The Eighth Doctor told her to sing it to help block out the sound of Aboresh and his brethren chanting. (AUDIO: Absolution)

When chased by a Feratu named Janet in a helicopter, Ace jokingly quoted the line from the poem, "Here comes a chopper to chop off your head!" (AUDIO: An Alien Werewolf in London)

Joe Buchanan once recited the rhyme in part, saying, "Here comes a candle to light you to bed, here comes a chopper to chop off your head". (TV: The God Complex)

Behind the scenes
It is also obliquely referenced by the episode title The Bells of Saint John, as one of the rhyming couplets from the original nursery rhyme goes "Pokers and tongs / say the bells of Saint John's".