The characters are now older than in The Machine Gunners, and the themes are appropriate to their age. The transition from boy to man is one of the central aspects of the novel, and Chas is ultimately faced with a decision a mature adult would find daunting.
"The Guns of Navarone" explores the themes of growing up, facing responsibility, and making difficult choices as characters mature.  Chas's dilemma is representative of the complexities adults face in their lives.