The episode title is a French phrase meaning "a passionate appeal, complaint, or protest"--literally "cry of heart."
The two photos Tony Armstrong-Jones gives to Her Majesty, meant to be a young Queen Elizabeth and Prince Phillip, are actually of Claire Foy and Matt Smith, the Queen and Prince from Seasons 1 and 2.
The episode won an Emmy Award for Outstanding Period Costumes.
The song playing when Margaret and Roddy are in the pool after shopping is Squeezebox by The Who. The song was written as a joke, "squeezebox" being slang both for the accordion (which Pete Townshend had recently purchased) and for a certain part of the female anatomy. Some have suggested it was written purely to see how far they could go and still get it past the censors, while The Who themselves have stated they never thought it would become popular and it was just a bit of fun.