Hi _rg_
The admin-en_GB mo and po files were in the wp-content/languages/admin folder of the site. After checking to make sure they were version 5.1.1 I downloaded and transferred new versions of these files and replaced the old versions with them.
Unfortunately, even after a restart, it hasn’t solved the problem. when changed to English (United Kingdom) and Saved Changes the Site Language reverts back to English (United States).
I don’t know if there is any implication in having a UK site with the Eglish United States as the Site Language and if there any other fixes I could try?