The words drifted across the frozen battlefield: 'Stille Nacht. Heilige Nacht'. To the ears of the British troops peering over their trench, the lyrics may have been unfamiliar but the haunting tune was unmistakable. After the last note a lone German infantryman appeared holding a small tree glowing with light. 'Merry Christmas. We not shoot, you not shoot.' It was just after dawn on a bitingly cold Christmas Day in 1914, ... and one of the most extraordinary incidents of the Great War was about to unfold. Weary men climbed hesitantly at first out of trenches and stumbled into no man's land. They shook hands, sang carols, lit each other's cigarettes, swapped tunic buttons and addresses and, most famously, played football. The unauthorised Christmas truce spread across much of the 500-mile Western Front where more than a million men were encamped. There is only one man in the world still alive who spent 25 December 1914 serving in a conflict that left 31 million people dead, wounded or missing. Alfred Anderson was 18 at the time. 'I remember the silence, the eerie sound of silence,' he said. 'Only the guards were on duty. We all went outside the farm buildings and just stood listening. But there was a dead silence that morning, right across the land as far as you could see. We shouted Merry Christmas", even though nobody felt merry.' In some parts of the front, the ceasefire lasted several weeks. 'I'll give Christmas Day 1914 a brief thought, as I do every year. And I'll think about all my friends who never made it home. But it's too sad to think too much about it. Far too sad,' he said, his head bowed and his eyes filled with tears.