The Upgrade Enable is set to Yes, but I did manage to find a way to make it work.
It appears to be a timing issue. If you can click on the Choose File button within a second of the new page displaying the button works. Any longer and the button greys out. Once you have selected the upgrade file you have about the same time frame to hit the Upgrade button before that greys out - strange behaviour!
Anyway, I have now upgraded to 1.2.1 (004) although the behaviour on the Firmware Upgrade page appears to be the same as before. I'll need to be quick to get the next upgrade installed when the time comes.