It’s 1.15 a.m. Connie Bowskill should be asleep. Instead, she’s logging on to a property website in search of a particular house: 11 Bentley Grove, Cambridge. She knows it’s for sale; she saw the estate agent’s board in the front garden less than six hours ago.

Soon Connie is clicking on the ‘Virtual Tour’ button, keen to see the inside of 11 Bentley Grove and put her mind at rest once and for all. She finds herself looking at a scene from a nightmare: in the living room, in the middle of the carpet, there’s a woman lying face down in a huge pool of blood. In shock, Connie wakes her husband Kit. But when Kit sits down at the computer to take a look, he sees no dead body, only a pristine beige carpet in a perfectly ordinary room …