Well done Damo!
I did my weekly 5k time trial today and missed the 24 barrier by 14 seconds - still a PB by 25 seconds though.
You seem to be improving exponentially - it's taken me a couple of months' hard work to move from my first sub 25 to be in reach of sub 24!
Out of interest my splits today were:
4:51
5:02
4:41
4:58
4:42
The second k included an incline which explains why it was a bit slower, I was tiring in the penultimate k, but found a bit in the last k to speed up going for the PB. Thought I might get closer to 24 but was staggering by the end
I don't think some endurance work would do any harm, can't remember if you said you do a long run every week. Whatever you're doing seems to be working pretty well, so throwing a few other things might help even more. Maybe some speedwork would help you go even faster than 4:30 pace and thus improve the times even further.