A Quest for Clemency — Cecilia Peartree

'A Quest for Clemency' is the fourth in a series of mystery / thriller novels by Cecilia Peartree. It is set in 1950s Britain, mostly in Cambridge.
Clemency, Oliver Quest's younger sister, gets her own story after playing a supporting rôle in the previous novels in the series. After she has an unsettling encounter with a stranger on a train on her way back to Cambridge, a series of unexpected events begins. Clemency and her colleague Dr Andrew Neville, a mathematician specialising in the new field of computers, are the targets of apparently random attacks from several directions. Andrew is reluctant to use the skills he acquired in a previous life to help solve the problem. while Clemency will employ anything that comes to hand - her shoes, a coal shovel, her sharp mathematical brain - to fight her way out of trouble.