Sebastian's comment, "In the eternal words of Wales' finest poet, 'It's not unusual ...' " is a reference to the singer, Tom Jones.
The title comes from lines in Shakespeare's "The Comedy of Errors", Act 2 Scene 2 - "... Until I know this sure uncertainty / I'll entertain the offered fallacy."
The address of Mrs. Merchant is "2B Arnott", an obvious reference to the famous Shakespearean line "To be or not to be..." from Hamlet Act III, Scene 1. Luella mentions the address in Act I of this episode and the "Arnott" street sign, as well as house number, are visible in two different scenes of Act II.
Two of the characters bear the surname "Merchant". "The Merchant of Venice" is a famous Shakespearean work.
In a very early scene, as Sebastian hangs up the phone, he says "What fools these mortals be" which is a quote from A Midsummer Night's Dream.