Once again, I seem to be in exactly the same position as Scrump - I have no musical training at all, but (usually) find it easy to play the chords to folk tunes/songs that I've never heard before, particulary when I'm part of a band. Yet I have a friend who has played first baritone horn in a brass band since she was about 15. To this day, she cannot play a scale unless it is written down. I find that incomprensible.

To go back to a point I made earlier, surely you don't need to be 'musically gifted' to start playing a song in the key you've chosen and realise somewhere in the first line, it's time to change chord because the original one isn't 'fitting' anymore. When that point comes, the choice is limited - it's almost certainly going to be one of two chords (or perhaps three.)Isn't it?