Many thanks to all for the prompt responses. RE-booted today and all works fine - so I am somewhat bewildered.
However, to avoid unnecessary posts I will class this as solved and cross my fingers! Should there be a recurrence I will refer to these posts for guidance.
Again thanks to all who responded.
Just installed Antergos (very straightforward) but when I reboot I get this message
An error detected in the current theme that could interfere with the system login process…
Followed by the 3 options : Load default theme; Load fallback theme; Cancel.
Choosing any of these has no effect at all so I am unable to look at any aspect of the system. I have read previous posts re this error but all were able to proceed with one of the three options.
This is my first time using a rolling release Linux OS and was quite impressed until this problem occurred. Any suggestions, please.