Herbert Howells is in my view the most under-rated British composer of the 20th century. As well as this wondrous carol and a host of other works he also wrote the stirring tune to my favourite hymn, All My Hope on God is Founded.
A Spotless Rose is dedicated to Peter Noyce, who took over from Colin Howard as St Mary's choirmaster in 1979 and who worshipped Howells with something approaching reverence. At one choir practice he described the end of this carol as "probably the greatest single bar of music ever written." I wouldn't quite go that far, but I know what he means.