The Software Update program checks what is available against what is in its receipts folder, as the names of the files are the same for 4L5/4L7 and 4L13 (the official update), it thinks your system is already updated. To resolve this, go to Library/Receipts and remove any ofe the pkgs from 4L5 and 4L7, then run software Update again... viola!
Well, my receipts folder is empty, and I have tried software update numerous times over the past day, and every time I get the software up to date message. I suppose the could just be REALLY busy, but... Oh well. That's what I get for trying to be too up to date, I suppose.
Thanks for your help, but unfortunately It's still not working for me. I'll probably try reinstalling if it doesn't start working shortly. Or maybe I'll just wait until I can get the update without using Software Update. Bound to happen eventually.