Several characters mention the year in this episode. Apparently the story takes place in 1922, which is strange as the novel was published in 1928 and Dorothy L. Sayers had the habit of letting her main characters age naturally with each novel. (Even more peculiarly, Clouds of Witness, the previous serial, was set in 1928, though the novel was published in 1926.) In the television time line these events take place before those of Clouds of Witness, in the novels it was the other way around.
Lord Peter: What an acid man you are, Mr Pritchard. No reverence, no faith. Do lawyers ever go to heaven?
Lord Peter [ to Robert Fentiman ]: After all you experienced during the war you oughtn't be so sensitive about corpses. You and I have seen many things far less seemly than a nice, quiet little resurrection in a respectable cemetery.