To signal soldiers to go over the top in World War I, an officer blew a whistle. For the film Warhorse, director Steven Spielberg chose his military historian adviser Andy Robertshaw. But this was someone who had already had a trench dug in his back garden by soldiers back from Afghanistan. Andy has been on Time Team and runs the Royal Logistics Corps Museum in Deepcut, Surrey. He explains how he made trench warfare authentic in this children’s story of a horse which worked for both sides.