Well I took the plunge and upgraded from 3.71A to 3.70E...
And it's bricked. I'm guessing it's similar to the issues people were having switching from some of the earlier beta bioses to 3.71A. I should have really downgraded to an earlier BIOS before I upgraded, oh well!
Unless it was just a bad transfer, the progress bar only got to 40% before it said it was successful and to reboot, can't remember if that's normal from previous attempts.
Now, I've ordered a X300 and will try to fix the A300 at a later date and sell it on, perhaps this was Asrock's plan all along?