Complex sentence: Smiley recalls that Leipzig had often used a venal Soviet agent named Oleg Kirov, who was susceptible to blackmail, as a source of information and surmises that Kirov is probably the other man in the photograph.
Split into simpler sentences: Smiley recalls that Leipzig had often used a venal Soviet agent named Oleg Kirov, who was susceptible to blackmail, as a source of information. Smiley surmises that Kirov is probably the other man in the photograph.