Once the update failed I could no longer launch Photoshop CS6. I got a dialogue box that popped up saying "Adobe Photoshop CS6 has stopped working." I tried a number of things, including trying to reinstall Photoshop CS6. Maybe I did something wrong but I couldn't get any joy. Luckily, I had a recent Drive image which I restored and got my lovey Photoshop back again.
I then did as jbierrie suggested and attempted to update from within Photoshop itself. Unfortunately, the same horrible result. Have just restored my drive again and will not try to install update 13.1.2 until I know that the problem has been addressed and fixed by Adobe.
I would love to know if other users are able to install the update without any of these issues.