Until one of her daughters hired a genealogist to create a family tree for her 70th birthday, Kate Atkinson always believed her grandfather to have been among 45 miners killed at Bentley Colliery in Doncaster, following a devastating blast in 1931.

“I’ve seen the grave,” she says. “It’s very moving. All the men were buried next to their best friends. Every year, they still have a ceremony to mark the tragedy.”

Then,…