After doing about 10 failed install attempts and waiting about 2 months I tried the AU again today. It failed again and put back my Version 1511 Build 10586.633 system OK. Rather than take the 7 minutes required to restore the Macrium Reflect boot disk image I made before I tried the update I just deleted all the update files from my boot drive.

And yes, I've tried (repeatedly) all the standard things to fix the update. Looks like I'll be on 1511 until I decide to do a clean install - which may never happen. My 1511 has been running solid as a rock for many months.

"If it ain't broke don't fix it" is making a lot of sense to me these days.