A Turn in the Lane — M.K. Jacobs

The Assistant Registrar Emily Price, from the London Archives commits a crime of passion. Or is it? A man falls from the balcony. Was he pushed? And what is Emily Price hiding? Chief Police Inspector Ralph McGinniss from Scotland Yard investigates. He calls in his newly appointed assistant for help.
Claire Lane is a Professor of Mathematics at London's prestigious St Anne's University. Her speciality-scientific study of probability. In other words, the likelihood that an event will reoccur.
She takes on the challenge until she discovers there is more to this story. A birth certificate surfaces, convincing Claire she is looking in the wrong direction. Another murder in Cornwall. Will the killer strike again and what will happen to Ralph McGinniss who discovers the truth.