“The whole kit in the bin…” I can surely relate to those moments? 
I’m not sure what you’ve done so far, but the process should actually be fairly simple (it always is if you know what to do)? You should start to download and install the latest Steinberg elicencer program:
Normally it shouldn’t be necessary but just to rule out anything that could cause an issue just restart your system after installing this.
After the restart make sure you’re internet connection is active!
- Now start the elicencer and cancel all the ‘maintenance’ and other messages until you get to the main screen.
- Click on the green arrow that says ‘Enter activation code’ and copy and past the activation code that was send to you here and click ‘Continue’.
- Wait until it says that it’s activated. After this you can close the elicenser.
- If you haven’t done so yet, install Cubase AI and after that you should be ready to go!