I'm coming to this very late in the day, having spent the Christmas holiday far from the internet (makes a change!). A solid day's work sees me still needing 2D (there's an obvious answer but I can't get the wordplay to work) and 17A which leaves me baffled. Am I right in thinking that once the unclued light has been worked out (done that) and the 4-letter coincidence found (done that) then the tedious business of gazing at pairs is unnecessary? And in my house I have many examples of the central coincidence: in some the longer part completely obscures the shorter while in other the shorter, being fatter, remains visible. I'm inclined to enter the central column in the manner of the second possibility - what do others think? (And a pointer to the two elusive clues would be welcome.)