Most surprised that I received the following email from Google!
I have no idea of the incorrect SSL Certificate origin.
Yesterday I deleted the SSL and installed LetsEncrypt again without success.
Today it works! It looks as though overnight the faulty URL has been de-propagated/un-propagated (?)
Lessons to be learned:
Do not use 301 to rewrite the HTTP URL to HTTPS because if there are any mistakes it takes forever to revert. Far better to use a 302 until the URL has proved to be successful. The rigid 301 change can then be made or maybe never?.
Beware of Ubuntu upgrades from 14.04 to 16.01 which may have been the problem, if not then it was co-incidental the HTTPS URL stopped working.
Test your site with a free URL and monitor success. I am delighted to say has worked fine otherwise my other sites would have been down for well over a fortnight.