I wasn't really convinced by this clue. I suppose that "Barer of glad tidings" could be an oblique definition, and the "(using central Post Office)" indicates that the answer is "stripogram" rather than "strippergram", but I'd have preferred either a full wordplay, or just the oblique definition - and should the word "Barer" be in inverted commas to indicate the pun?
So I propose the following alternative version of the clue for 27ac: 'Barer' of glad tidings? (10)
I wonder whether this is a new setter. This week, I finished the Listener Crossword before finishing the "easier" Times Crossword, as some of the clues took a bit of thinking through.