Just after the introduction, when the naturalist's office is shown for the first time, sounds of typing can be heard in the background. They actually form the monotonic rendition of the intro music.
The character of Bennett has the forename Jack, in Doyle's short story his name is Trevor.
Michael Cox: Being the last episode of the series to be involved with as producer, he performs a cameo as zoo cleaner in the background of the zoo scene where Lestrade talks to the zoo director. His face is barely recognizable, but this event is mentioned in "A Study in Celluloid: A Producer's Account of Jeremy Brett as Sherlock Holms" by Michael Cox, 1999, Rupert Books Cambridge.