I finally ditched the old SEO plugin I'd installed years ago when I actually cared about precisely formatted page descriptions, Twitter Cards and the like.
I'm over all that now!
The only reason I'd kept it this long is because, for a supposedly "non invasive" plugin, it altered post layouts and everything had been styled based on this.
So, biting the bullet, I deactivated the plugin and sorted the layout issues that caused.
It also meant that certain sections of the page header (such as the Twitter Cards and Open Graph markup) were now missing data so I've just decided to remove them. It's left the page header much cleaner and I just don't worry about those sorts of things any more.