As a relative newcomer to the Listener, I occasionally find myself hitting a brick wall. Such is the case on this occasion.
I have all but one answers to the clues, and have the grid appropriately filled. The theme and the 17s came to me quite by accident but early on.
What I do not have is 16A and any idea of how to reconcile the letters missing from certain answers. Is it in some way relevant that there is no mention of these missing letters in the preamble?