Initially the singer and guitarist. They started as a duo. Eventually they got the three other blokes to turn it into a band.
Then Greg, the flute/kbd/sax player also wrote. I don't recall the other two writing except for John, the bass player, may have written and sung one, possibly two songs.
But pretty sure Colin, the singer was the main writer. He was really clever, and very artistic even for a Scotsman! [but grew up here in MEL].