Last Saturday, 20 December, I went back to my old school, Loretto, for the first time in 29 years. I was drawn not by any great feeling of nostalgia for my schooldays, but because the story of my great-grandfather, Archibald Henry Buchanan-Dunlop,  was the impetus behind events that were being held there to mark the centenary of the Christmas truce on the Western Front. These culminated in the unveiling, in the school chapel, of a window made by Kate Henderson commemorating the truce and my great-grandfather’s role in it. But what, exactly, was his role?