As I know you're the same age as me I didn't need to look it up. I rather like this song. Originally called Jenny apparently. Acker Bilk wrote it for his daughter.
I was looking at 1977 where they say Rod Stewart was the biggest seller, and I thought it might be that old dispute where in fact Sex Pistols' "God Save The Queen" beat him to #1 but it was banned. However it was the wrong Rod Stewart song listed.
Then I thought surely "Mull Of Kintyre" was the biggest hit in 1977, and the penny dropped. This article refers to the U.S. charts, Billboard as they're known.