The extraordinary story of Mary’s Meals: after watching a news bulletin about war-torn Bosnia, two brothers agreed to take a week’s hiatus from work to help. What neither of them expected is that what began as a one-off road trip in a beaten-up Land Rover rapidly grew to become Magnus’s life’s work – leading him to leave his job, sell his house and direct all his efforts into feeding thousands of the world’s poorest children. He talks to Sarah Crompton.