Hello Owen,
It's available via the 'Download' menu at the top of the main Hauptwerk website:
https://www.hauptwerk.com/win/
Although that page mentions trials, the software is the same in any case (it's the presence or absence of an iLok licence that determines whether Hauptwerk runs as a full version or a trial).
You also need to install iLok License Manager, which can be downloaded here:
https://ilok.com/