The exterior shots for Spellman's Museum of the Circus is the same location used for Warriors of Kudlak: Part One (2007)/Warriors of Kudlak: Part Two (2007). It was also used as the Odeon Theater in From Out of the Rain (2008), which also featured a circus of horror.
When Sarah Jane is looking at pictures of clowns on her computer, one of them is of Clara the Clown from The Celestial Toyroom (1966)/The Celestial Toymaker.
The Pied Piper of Hamelin was used as the theme of the 1965 Doctor Who comic strip adventure Challenge of the Piper starring the First Doctor.
The flashback of Sarah Jane's childhood of a clown doll moving during a storm is reminiscent of a scene in Poltergeist (1982).