Monty Don, the television gardening presenter, always comes across on screen as irrepressibly cheerful and enthusiastic, but this is a misleading impression. In fact, he gets black moods. ‘It’s no secret that for many years I’ve suffered from depression,’ he said last week at the Cheltenham Literary Festival. He had tried Prozac and cognitive behavioural therapy, but the only treatment that had worked for him was provided by his dog.