Hi,
The https://account.microsoft.com/services/ link provides the ability to see I have a license, and offers the ability to download the office software - but nothing about activation.

I have previously downloaded office (again) using that link in case it embedded the activation, but I just get the same error.
When I try to use the virtual agent to reactivate I just get told there is a problem (both with reactivating and getting a call back).

But looking at your second link, I now think I at least know what the problem is (or at least suspect what the problem is) based on the following:
"If you have a retail license and need to move it to a new computer, uninstall it from the old computer and install and activate it on the new computer. The retail licenses are limited to one computer and cannot be moved to new computers frequently like the subscription version, so deactivation is not necessary. You are limited in the number of times and how frequently you can reactivate it on new hardware (once every 90 days)."
Bolded for emphasis - because I bought and installed office within the last 90 days, it will not reactivate on a new PC, even if I have uninstalled it from the old one.
Which leads me to believe what I need to do is speak to support and get them to re-activate the license manually - however as you can see the agent isnt working. Is there a phone number I can directly call/email address to use, instead of going through the agent?
Thanks