The destruction of evidence

No evidence remained on the bodies of Alun Pitcher, a well-liked, wealthy, careful and cautious businessman, and his family after they'd been wrapped in brown paper and string, doused in petrol and ignited. The fires had destroyed any evidence, or so the pathologist and the local police believed. Defeated, the local force asks for outside help. Following success in solving a previous 'cold' case, Inspector Trevor Joseph and Sergeant Peter Collins are called in. Their superiors trust them to provide answers to the questions that have confounded the local force.