This is a most fascinating episode from Dorothy Sayers' "Lord Peter Wimsey" series and the second one produced for television with the wonderful Ian Carmichael, but understanding it requires piecing together its sometimes difficult and ambiguous chronology. In fact, an IMDb contributor correctly points out that terming the flu outbreak in this episode Spanish Flu is an anachronism as he sets the time frame as the late 1920s.
The 1918-1919 outbreak was one of the most horrific plagues ever, killing 40 to 80 million people worldwide. Although the 1918 pandemic is correctly termed Spanish Flu in Episode 1, by the end of 1919 it had mutated into a different, less virulent outbreak of influenza. The events of this episode and the two subsequent ones clearly occur in 1934, so having the characters use the term "Spanish Flu" adds to the already vague and hazy chronology.
This is logical as Dorothy Sayers' novel was published in 1934. Although no mention of that year is made in this chapter. it can clearly be extrapolated from the date given by the vicar, who says that the decomposed corpse dressed in prison garb and presumed to be Deacon was found in 1920, the year he and his wife moved in and two years after the real Deacon had killed a warder and escaped. Therefore, the disloyal butler had made good his jailbreak in 1918 after serving four years of his prison term.
That would date his conviction and jail term, as well as the robbery of the emerald necklace, back to 1914, four years earlier. That dovetails with the mention of Kaiser Wilhelm's "sabre rattling" at the wedding reception in Episode 1, and clearly refers to the German monarch's actions prior to the outbreak of hostilities on July 28, 1914.
During this episode, which takes place on New Year's Eve and Day and the subsequent Easter, it is mentioned several times that the robbery occurred twenty years earlier. If 20 is added to 1914, it adds up to 1934, the new year Sir Peter agrees to help bring in by tolling one of the churchbells as well as the year Sayers' mystery was first published.