The computer was showing a command line login. The SDDM login was failing to show.
Once logged in, if I waited a while and then pressed any key the SDDM login screen would appear. Username and password to be entered as usual.
Interestingly in the top bar it was the US layout which was selected (no others available).
I also found that entering the wrong password I was returned to the login screen once more, where the gb layout was shown.
Through searches on the Internet I found this reference
Whilst not my answer, the included reference to this article
provided the solution.
The recommendation was to add the haveged package.
First install the package: apt get install haveged
Whilst the recommendation was to enable and start haveged:
systemctl enable haveged systemctl start haveged
I chose to restart the computer to view whether the problem was persisting.
Problem solved! Albeit still with US layout.