Thanks @Parker1090 - I found it in the small hours of this morning.
Having seen that screenshot before, it looks as though “Invoice style gallery” should be in the dropdown for “Sales”.
Nowhere in any of the documentation, or at least not to my sleep deprived eyes last night, could I find where to find the button being described - it’d make sense to me for this to also appear as part of the invoice editor in settings.
Alas, all sorted now - onwards to try and make the invoice work with their blasted PDF printer.