Hi James, thanks very much for the advise. Reset the password option as you suggested and got no email asking to confirm which indicates “something on the server has changed”. Still mystified because our backed-up version of the site still functions fine on the test server.
I have installed the WP SMTP Plugin and thanks for that suggestion. Tempted to go for a completely fresh install to see if that rectifies things.
Thank you again for your help with this.