There is a classic literary problem that is mentioned in classes all over the world. When a character or plot seems meaningful and symbolic, does that automatically mean that the author intended that meaning? In this novel, the reader has an opportunity to dissect the character development of the post-apocalyptic survivors and their journey with a person whose very name points to his "strange" inclusion int he novel. Is Strangman a symbolic character, and if so, what does he symbolize?
To answer that question, look at the story before and after his appearance. His arrival is preceded by the team experiencing bad dreams. The dreams have a sublime quality that makes the team alert to potential changes. Then, Strangman, a man whose name underlines the strangeness or sublime nature of the moment, arrives and things do indeed change. For instance, he instantly gathers influence among some of the team, splitting the team into two factions; half the team like Strangman, and half are resistant to the change he brings.
Strangman and Dr. Bodkin are at odds with one another, and it isn't difficult to see why. Dr. Bodkin feels a great deal of responsibility for the team, and because he feels responsible for them, he manages the safety and order of the team among the chaos of the post-apocalypse. Dr. Bodkin doesn't like change because it threatens the order that his authority imposes. When Strangman drains the swamp, the reader sees the ultimate effect of his presence. Yes, he brings change—as any stranger would—but by accepting his influence, they discover the ruins of London, a symbol for the future of society reemerging from the waters of chaos. Progress will take them forward into new ways of life.