Thanks. Is it not available from Apple directly though?

It's right at the top of the Mac App Store as 'featured' with this story. Yet somehoe it doesn't include an install link!

https://apps.apple.com/gb/story/id1699686314

EDIT - COMMAND + R in the settings app will reload the page

(In Settings -> Software Update) if anyone else is having issues figuring it out.

That's just a "what's new". You can't install it from there.