Episode where they're all hiding in an office?

I was hoping someone could help me find the name of the episode where Mark and Steve are hiding in an office, waiting for the killer to arrive, and when the door opens, it's not the killer, and they hide again. This goes on 3 or 4 more times.
it's my mum who's looking for this episode on dvd, and I'm a huge fan, and I don't remember it, so I was hoping someone would. Thank you.

"I am alone with my sheep. My sheep are not alone with me." As time goes by.