I think I've gone wrong somewhere. From the letters in the down clues in 5d I need an A, but no word in that clue begins with an A.
I then thought it must be one of the clues that uses a symbol to replace some letters. The problem with that theory, though, is that the line from the poem given by the letters from extra words in the across clues is 17 letters long and there are 19 across clues - so it would seem both answers that use symbols come from the across clues (I've already got one).