One Is One And All Alone — Anthea Fraser

Malcolm Bennett, Webb's friend and colleague, has recently remarried, and there is conflict between his grown-up family and his new wife, making for a tense atmosphere at home.
Meanwhile, the two detectives have joined forces to tackle a series of shop raids, but are soon overtaken by events which have a shattering effect on Bennett's family. Two of them meet with a violent death within the space of four days, presenting Webb with one of the most traumatic cases of his career.