Doctor Who Series 10: 12 Easter Eggs & References You Might've Missed In "Knock Knock'

Here's a bit of fun. In early drafts of the script, it was revealed that Bill's housemate Harry - as played by Colin Ryan - is actually the grandson of Harry Sullivan, a companion of the Fourth Doctor!

The nod, however, was removed because, according to Mike Bartlett, "in 2017, people might not remember one companion from 40-odd years ago, so it got cut."

This reaction is strange, though, considering that Harry was already referenced in the preceding season. In 2015's The Zygon Inversion, it is stated that Harry created a gas for UNIT that could destroy the Zygons.

A reference to Harry's granddad visiting the Great Wall of China with his boyfriend did make it into the finished episode. However, sadly that granddad is not intended to be Harry Sullivan.