There could be nothing more sweet and sentimental than the sound of traditional carols performed by a velvet-voiced choir at Christmas. Or so you would think. Composer Howard Goodall uncovers the surprising and often secret history of the Christmas carol. Far from being accepted as part of the celebrations of Jesus's birth, over the centuries carols have been banned by both church and state. The carols we sing seem set in stone and yet they can have up to 400 regional variations. Individual carols have caused controversy - While Shepherds Watched had to be cleaned up by the Victorians for being too crude and there's a suspicion that O Come All Ye Faithful was a call to 18th century Jacobites to rebel. The documentary celebrates the enduring power of the carol with a variety of performances from folk singer Bella Hardy to the choir of Truro Cathedral.