The first time I heard this carol was fifty years ago. Hundreds of us had just been evacuated from London Bridge station one freezing cold December afternoon as a result of an IRA bomb warning (nothing came to pass). We were all standing around outside the station in shivering misery when a very small old lady suddenly burst into song with this carol. The wren is one of our smallest songbirds but it has the loudest and clearest song, a bit like that old lady. There could not have been a better way of cheering us up. That moment has stayed with me, especially at this time of year, down all those decades.