My thanks too for post 21.
Like others I have a completed grid (and have successfully reverse engineered the 4 changed clues - none of which had I solved before).
I remain a bit troubled by 16a - both the letter change (can only decipher the last 4 words of the message, although that was enough) and the word play, which would make more sense to me if the first two letters were swapped.
All rather enjoyable nonetheless