Many thanks for these replies!
I can now get into cPanel and also, my WP Admin page (albeit only in recovery mode).
I’ve now established it’s an error with the WP theme I used which is now obsolete (i.e. no support available for this theme anymore). I think I’ll need to disable the theme, start using a new one and rebuild the site, which is going to take time.
For now, I’ve had to use some ‘bush mechanics’ (as we say Down Under) and apply a redirect on the site. Hey ho!
