Unholy Murder by Lynda La Plante

"A coffin is dug up by builders in the grounds of an historic convent - inside is the body of a young nun In a city as old as London, the discovery is hardly surprising. But w hen scratch marks are found on the inside of the coffin lid, Detective Jane Tennison believes she has unearthed a mystery far darker than any she's investigated before. However, not everyone agrees. Tennison's superiors dismiss it as an historic cold case, and the Church seems desperate to conceal the facts from the investigation. It's clear that someone is hiding the truth, and perhaps even the killer. Tennison must pray she can find both - before they are buried forever.