See, I said he would know!
For the rest of you: It is from an episode of Dr Who. They spend a fortune on special effects (both practical and CGI) to produce aliens and monsters, but the most frightening was simply a child in school uniform, wearing a gas mask, who just keeps walking up to people and asking "Are you my mummy?"
Simple, cheap, and very basic - But if you have not seen the episodes it is hard to understand how unnerving the scenes were to watch.