I've finished this to my satisfaction except for 27across. I don't see how one can determine which the extra letter is, until one knows what the "sentence" is.
By the way the heading number is wrong. It should be 4351.
Can anyone explain the wordplay in the following clues please? I have a filled grid.
12a A former group concerned with writing? (3)
2d Mobile home impounded by city officer creates a thorny picture? (8, two words)
Thanks