Waterland by Graham Swift.

Tom Crick, fifty-two years old, has been history master for some thirty years in a private secondary school in Greenwich, a point of zero degrees longitude, in a sense the place where, in a world that sets its clocks according to Greenwich Mean Time, time begins. Tom’s wife, Mary, also in her early fifties, has been married to Tom for as long as he has been teaching. Until shortly before the immediate action of the story, she has been working with the elderly in a home. She has given up that job.