SOLUTION!
Just in case there’s anyone out there reading my posts apart from me:
Just got Twitter feed working again!
I thought I’d try disabling Wordpress plugins one by one. Should have done it in the first place!
When I disabled a plugin called ‘HumansNotBots - Easy, Accessible Email Cloaker’ - the Twitter feed re-appeared!
It’s still a mystery as to why that plugin is the problem. As I said, at the point when the Twitter feed stopped working, I hadn’t done anything. I hadn’t updated Wordpress or updated any of the plugins. Plus, the site had been running for several months with all the plugins working including the one above.
I’d still be interested if anyone has an explanation?