Yeah, I installed it last night and I've just opened App Store and it's there again. Just came here to see if Apple have just pushed out an update to the update, but App Store still says 1.2 😕
I'll install it again and see what happens.
EDIT: OK, same thing happened, I've discovered that when it says you must be connected to AC power, it really means it. When it restarts and you get the white screen with the grey Apple logo and the progress bar at the bottom, if you aren't connected to AC it immediately skips to the reboot - I thought originally that because it was such a small download that it installed so quickly you didn't see the progress bar move, but when I tried a third time with AC power it took about 20 seconds to install.