Michael Morpurgo is best known as a successful and much-loved children's author and the creator of 'War Horse'. Yet he describes his "greatest story" as the rural charity he set up with his wife Clare in 1976. This year Farms for City Children celebrates its 40th anniversary and to mark the occasion we visit Michael and Clare on the isolated Devon farm where the project began. Over the last four decades almost 100,000 children from urban schools have pulled on their wellies to experience the 'muck and magic' of life on a working farm and discovered what it means to feed and care for livestock. Now there are two other farms, one in Pembrokeshire and another in Gloucestershire, providing week-long residential stays. But what challenges face the charity today and what are the prospects for the next 40 years? Presented by Charlotte Smith and produced by Vernon Harwood.