So, I have done the following:
Updated the License Manager
Made sure that this computer (OSX Catalina) is authorised on my Line 6 account
Been sure the HX Edit is the latest version
Made sure that HX Edit logged in to the same account that purchased the IRs
Bought another set of cheap IRs to be on the safe side
Updated the HX Stomp firmware from 2.90 to 2.91
All I can think now is that this is an authorisation issue between the Line 6 purchasing account and the account signed in to HX Edit, but having logged in and out of both a few times, there is only one account and HX Edit signed in to the one that bought the IRs.
Unless anyone else has any ideas, I think this is a support ticket issue...?