Although a lot of the performers involved in the series were well-known, and many went on to achieve greater fame post-series, Wilfrid Brambell was almost inarguably the most famous guest actor at the time it was made. His status appears to be reflected in the credits, where, although listed last, he appears on a credit by himself, with an "and" before his name, seemingly indicating a star status.
Wilfrid Brambell gets a small role as a man who claims to be "78", yet Brambell was just 72 in real life when it was filmed. Looking notably frail, he died less than four months after the broadcast of the episode.
Kelly, his co-writer and his producer all end up in the position of the "three wise monkeys" at the close of the episode.