Berlin Fall (The Blind Sleuth Mysteries Book 8). The Blind Sleuth Mysteries, #8

While treating a patient in the fall of 1972, Daisy managed to winkle out of him that he worked for MI6. Then she blabbed about a planned visit to East Berlin with her friend Margery, who was a chemistry researcher at King's College. Back at the office, the man asked his spooks to do some background checks. It turned out that without even knowing it his blind physiotherapist and her chum had an indirect connection to a high-ranking communist party boss.Meanwhile, in East Berlin, clever operatives of the GDR secret services realized that Margery must know some pretty vital scientific secrets.
They decided to put Hans Konradi on the case during the visit of the two Englishwomen to Ost. Young Hans was not an agent, just a charming student with fluent English who could easily be pressured into spying for his country. But Hans had an agenda of his own, and 'Operation Berlin Fall' did not turn out the way the spymasters on both sides of the Wall had envisioned.