Ethereum: Cannot use master password on Ethereum wallet
I never managed to solve this particular problem, however I'm now using Mist to great effect.

I found that it was possible to move keystore files from one installation to the new Mist installation and use the passwords I originally created with minimal fuss.

So, my suggestion to anyone having similar problems is:  


Backup the keystores: %appdata%\Ethereum\Keystore
Uninstall Alethzero (seriously, I still don't understand what the hell that software even is!)
install the official Mist client, which is beautiful and slick, much easier to understand and can be found here: 
https://github.com/ethereum/mist/releases


You should then be able to copy your keystore files back over - each seems to represent an account and will have a password attached. Mist will just pick them up when you start - and you'll need the password for each account when you attempt to use it.

Best of luck.