Yo Martin.
Sometimes defining when a song is actually finished is an inexact science.
Is it when you have a complete melody,but not all of the lyrics?
Is it when you have some lyrics and some idea of melody?
Is it when you have a complete lyric and struggle to fit a melody?
Is it when you have a complete song,but haven't recorded it yet?
Is it recorded,but still a work in progress on the production and arranging front?
Then,there are those songs that you imagine are finished and you suddenly get an idea you want to add,or change.
Generally,if I get a song mastered,I'm done with it.
As Paul said,we all work differently towards the same goal. I usually have a bank of lyrics,or lyric lines/ideas. I scribble these down on random bits of paper when they occur to me. While playing the guitar,a riff or a chord progression may come along and develop into the early structure of a song.
Normally,I'd say I'm similar to Paul,as I work at a piece until it's at least a whole song...finished may be a while yet.
Lately,because of the advent of Logic in my life,I may have several songs in various states of repair,simply because a daw allows you to do this. So the way I write is evolving and including anything that gets the juices flowing.
I hope I muddied the waters a little more with all that bollocks.