Ah! My 3rd line may have been perfectly standard L fare, but "each" is not "even ". It all makes sense now, so thanks to all, as ever.
The denouement was predictable from the nature of the "correct" letters for the third line.
While it's quite clever, I really cannot join in with all the praise - I think this would actually have been one of the easier L crosswords to set. In addition, the letters in the third line were unchecked, making it relatively easy to make the sort of blunder I made (and also shows that I'm not much better at doing the crosswords than I was 20 years ago when I started doing them).