Hi, rrrobbo.
13 of the 15 missing letters are checked. In both the across and the intersecting down clues, these letters are omitted in the wordplay of both the across and the down clues. The intersection of 1d and 12a is a good example of this - the letter at the intersection is in the thematic phrase, and so is omitted from the wordplay for both clues.
The remaining 2 missing letters are unchecked. They are both in across clues in the lower half of the grid.
Several clues contain more than one omitted letter.