In the series, the idea was made on the concept that MI5 officers have temptations to make money. The written scenes where Salter confesses to Tom was somewhat inspired by Tinker, Tailor, Soldier, Spy. The scene was for the two to act as friends rather than interrogator and suspect.
The concept underlying the series revolved around the idea that MI5 officers have a weakness for personal gain, which was influenced by the novel Tinker, Tailor, Soldier, Spy. The scene where Salter confesses to Tom was written as an opportunity for them to act as friends rather than adversaries.