I'm posting in case this helps anyone else, although I'm not sure if my inablillity to copy music was to do with the OS... or my own ignorance...
Following advice in previous posts, I deleted apps to make sure I had enough space, I tried syncing... several times, I tried backing up then restoring iPhone from backup... but still a new playlist want't copying from iTunes to iPhone. I tried the sharing option, but as other people have pointed out as soon as I close iTunes on my macbook, the new playlist disappears from my iPhone.
Then (in iTunes on my macbook) I dragged and dropped the playlist onto iPhone and it has now sucessfully copied the tracks and the playlist. The only difference between everything I tried earlier and the drag and drop I've just used is that I removed the data cable connecting iPhone to Macbook (after I enabled it for syncing via wifi). I don't know why it has worked this time, but it has.
And the reason I mentioned my own ignorance... I didn't know you could just drag and drop from one to the other, I thought iTunes only worked by syncing content between devices :/