What phrase/idiom can be used to refer to procrastination?
Solution 1:
You've let it slide, you've been dragging your heels, you've been putting it off.
Solution 2:
Sorry for this belated reply, but the word is 'belated'. ;-)
Solution 3:
'Sorry for letting the grass grow under my feet' is probably going to deflect their annoyance by the mild humour involved.
Solution 4:
use delayed, it is professional and means, well, not on time