Suppose I'm replying to someone's email and it's long overdue - completely hypothetically :)
Starting with "Sorry for [ ... ] this". At first I thought I'd use "dropping the ball on" but it seems that "dropping the ball" refers more to making a blunder than to simply putting something off for too long.
Any ideas?
You've let it slide, you've been dragging your heels, you've been putting it off.
Sorry for this belated reply, but the word is 'belated'. ;-)
'Sorry for letting the grass grow under my feet' is probably going to deflect their annoyance by the mild humour involved.
use delayed, it is professional and means, well, not on time
It depends on the audience. It may be especially important to get the tone right in this situation, because it's possible that your recipient is upset, and you need to mollify them.
For instance, in a business letter, you might start with something like "I apologise for the long delay", and continue with a respectful tone throughout (without being obsequious). In a letter to a friend, something more light and humorous could work, but if the friend is really upset, florid apologies might be called for.
It may be worthwhile to try to think of something original here, since using stock phrases may not improve your correspondent's mood.
