The First World War Armistice (the agreement to end hostilities between Germany and the Allied Powers) was signed on November 11, 1918 in a railway carriage in the Compiègne Forest in northern France.
The signatories were Marshall Ferdinand Foch, representing the Allies, and the politician Matthias Erzberger, representing Germany.
The poet Wilfred Owen was killed in action on 4 November 1918, exactly one week (almost to the hour) before the Armistice. It is likely Muriel Spark was aware of this.
Armistice Day in London: