Complex sentence: It is the band's best known and most commercially successful recording and was the 1981 Christmas number one in the UK, where it has since sold over 1,560,000 copies, making it the 23rd most successful single in UK Singles Chart history.
Split into simpler sentences: It is the band's best known and most commercially successful recording to date. In 1981 it was the Christmas number one in the UK, where it has since sold over 1,560,000 copies, making it the 23rd most successful single in UK Singles Chart history.