Hallo Swart
Thanks for the reminder that ppa 's can be the source of these woes. 
The offending ppa 's have been identified by a process of trial and error. They were:
(A)
ppa.launchpad.net/ffdiaporamateam/stable/ubuntu xenial main
(B)
ppa.launchpad.net/accessibility-dev/ppa/ubuntu xenial main
I removed them and the update process run properly once again.