It's not topic pinning, that last run at 11:49am and is set to be every 6 hours, so 5:49pm will be the next time for that.

Delayed inserts are intentional and normal. We actually do this for performance reasons now, it prevents locks, and hence deadlock situations. Typically these will be stat logging or other similar logging that doesn't need to be kept up-to-date.
Curiously the same Connection identifier seems to be being recycled for different delayed inserts. The time is resetting so I'm pretty sure they are different ones. I'll look into that, it's at least interesting.

I will see if I can find any logged errors.

Okay, I think the persistent IDs is because they're thread IDs, and there's an internal thread pool
http://dev.mysql.com/doc/refman/5.5/en/thread-pool-operation.html
i.e. threads are re-used. It's not the same query getting stuck.

Okay, so there's nothing in the error log. It would be interesting to see what critical error was reported, but at this point (without doing a major investigation) I can't nail down any cause. MySQL usage looks low and there are no reported locks. I wonder if perhaps it was a traffic spike, e.g. bots.

I think that you might of been right about bots, as it has not reoccurred.
I will keep an eye on it. I did capture the critical error and entered it in to a tracker, but i then think I must of hit the wrong button because it never appeared :-( and I didn't keep a copy !!