One of the rules of the series is that an actor can't appear twice. Derek Jacobi's voice was in The Devil of Christmas, but as he was never seen physically he appears in this episode.
Presumably the names "Urban" and "Webster" are a little Dictionary joke, especially since Webster is much more likely to come out with profanities.
When consoling Webster, Urban tells him, "When you do something unforgivable, forgive yourself" - a line of dialogue taken verbatim from David Mamet's script for the 1987 film "House Of Games".
The relationship between Urban and Webster is strongly reminiscent of that between Norman and "Sir" in Ronald Harwood's play "The Dresser" (filmed in 1983).