Much like s_pugh, I had everything done bar the theme and then got stuck for a while. Was encouraged by posts here to step back and look again, saw an obvious answer from the unclued entry, but still wasted a lot of time trying to deduce it from the instructions given.
One of these seemed to suggest that there were two types of 45 50, and I wondered about 'directly shown' v 'indirectly suggested', but this still gave more than 13 letters and a couple of 'which category does this fit?' There was also the chance of a different approach located within that one clue, and most of the letters were there, but it didn't quite hang together.
When I finally spotted what was required, I was slightly annoyed. It only makes sense if you read the instructions out of order! Still, a minor quibble given the elegance of the whole thing. Quite an easy puzzle, but some real detective work needed to unravel some of the wordplay, leading to discovery of some words I'd never heard of. I have to say, though, that I'm not a great fan of clues which break down into a string of one- or two-letter abbreviations!