who died at 65 early Thursday — the story behind his band’s Christmas-in-the-drunk-tank classic “Fairytale of New York” began with Elvis Costello.The “Alison” singer was the Pogues’ producer at the time, and he bet the band that they couldn’t write a hit holiday single.But MacGowan and Pogues banjo player Jem Finer would prove Costello wrong after writing “Fairytale of New York” — the most-played Christmas song of the 21st century in the UK — in 1985.Finer came up with the melody and original concept for the song, which was initially about a homesick sailor on Ireland’s West Coast. But Finer’s wife suggested changing up the lyrics to be a conversation between a couple at Christmas.Then the notoriously hard-drinking MacGowan took it from there and transformed it into the Broadway-style tinsel tune we know today.“I sat down, opened the sherry, got the peanuts out and pretended it was Christmas,” he told Melody Maker in 1985.“It’s quite sloppy ….