“We never got to tie up John,” says screenwriter and producer Matthew Graham, reflecting on the premature end of BBC One’s hit sci-fi police procedural Life on Mars.

In sequel series Ashes to Ashes, the writer-producers managed to do everything they’d planned for Philip Glenister’s gruff, outmoded DCI Gene Hunt, but not for John Simm’s DI Sam Tyler. Why? “John had gone by then,” Graham tells Den of Geek in a new interview.

Life on Mars finished in 2007 after two series but the original plan, Graham says, was to continue for a third and possibly fourth run. “We had one of the biggest hits on television,” he tells us, “[…] the last thing you want to do is come up with a hit show and then say, ‘right, I’m pulling the plug on it’.”

Star John Simm though, asked to leave. “He was obviously in every scene, he was very tired, he’d just had a new baby, he was away a lot from home and he just came to us and said ‘look, I want to call it a day’.” That happened midway through filming the second season, Graham remembers, “so it was a bit of a last-minute thing.”