Who are the only artists to have reached the number one spot at Christmas with the same song, in the UK, on two separate occasions?

"Bohemian Rhapsody" by Queen, which reached the number-one spot at Christmas 1975 and 1991, is the only record to have reached the top twice. "Mary's Boy Child" is the only song to be Christmas number one for two entirely unrelated artists (Harry Belafonte in 1957 and Boney M. in 1978), although "Do They Know It's Christmas?" has been Christmas number one for three generations of Band Aid. The original version of "Do They Know It's Christmas?" is the second best selling single in UK history.