Boney M. were huge:

Boney M. are the only artists to appear twice in the top 11 best selling singles of all time in the UK, with Rivers of Babylon in seventh place and Mary's Boy Child/Oh My Lord at number 11. They are also one of six artists to sell a million copies with two singles in the same year.

And that's from Wikipedia, so it must be true.

I was pleased when Mary's Boy Child turned up in the charts because I once sang it at a primary schools' music festival. Plus "And man will live for evermore because of Christmas Day" was one of the lines that did it for me in those days.

Mind you, it was even more impressive when Cat Stevens took Morning Has Broken from our school hymn book into the top 10.