Tens of thousands of soldiers went 'over the top' at 7.30am on 1 July 1916 on the first day of the Battle of the Somme. Nearly 20,000 British soldiers died that day, which looms large in the collective national memory of the First World War. Cameramen Geoffrey Malins and John McDowell were there to record footage that became the cinematic sensation, Battle of the Somme.