Very neat grid, interesting device, and some lovely clues. A few less lovely clues, the preamble vagueness about “across clues’ thematic items”, obscurity of the source and flailing around for a quotation slightly spoiled it for me: it came down to gwit/solve-by-google, which took the edge off what should have been delight at the way it was represented in the grid! But still an enjoyable solve: thanks Leo.
45d is all checked letters, but I can’t figure out that clue. Could someone possibly give me a wee hint to set my mind at rest?