Snake in the Grass — Anthony Gilbert

Con Gardiner had no family; his work and his one-room flat filled most of his solitary existence, until one evening a strange girl in the street asked him to lend her a pound.

Con was attracted to Caro Graves, and puzzled too; he couldn't see what would become of this girl who had just left her husband after a bitter quarrel, and who had nowhere to go. But he was soon to have more to worry about: Caro's husband was dead . . . and Caro was the main suspect.