At the end, Cowley called Batak 'Great Uncle Bulgaria', an obvious nod to The Wombles.
This is the second of two guest appearances by Carol Royle. In both episodes she plays the illegitimate daughter of a recently-deceased Englishwoman and an Eastern European spy who arrives in the UK during the episode.
The tune that Anna Hastings/Batak plays on the piano which Doyle hears before introducing himself to her as a reporter is the Mazurka in C-sharp minor, Op. 63, No. 3 by Frédéric Chopin (1810 - 1849). This was written in 1846 and then published in 1847.