I suspect that Tam Lin is a book that will benefit from re-reading. Even knowing the ballad and therefore expecting the plot to pan out the way it did, the shape of the story is obscured until you have got to the end (though there were bits I worked out in advance, like looking up the Wikipedia article for 'When That I Was and a Little Tiny Boy' and realising who Robin and Nick were) and I'm sure a re-read would show all kinds of clues that are there for those who really know to look for them.