Writer Sally Wainwright based the story on the experience of her mother Dorothy who was put in touch with her childhood sweetheart Alec via Friends Reunited after being widowed at the age of seventy-five. Alec had also recently been widowed. The couple fell in love all over again and got married.
Creator and writer Sally Wainwright was so impressed by Sarah Lancashire's portrayal of Caroline that she devised Happy Valley (2014) as a solo project specifically for Lancashire.
This show was titled "Antony & Cleopatra" for a few months before being renamed.
Sir Derek Jacobi (in Love Is the Devil: Study for a Portrait of Francis Bacon (1998)) and Anne Reid (in Recomeçar (2003)) have each played characters that have had affairs with a character played by Daniel Craig.
In the opening title sequence, the "family tree" shows each actor's or actress's birth date (day and month, no year) under his or her name. Caroline (Celia's daughter) and Gillian (Alan's daughter) are supposed to have the same birthday, which is brought up specifically in the series. Both show different months under their names in the credits. One would assume that these then are the birthdays of the actors in question, but none of them are actually correct and seem to be a poetic flourish.