Good evening Allistair and welcome to the Events Calendar Support forum!
Thank you for reaching out to us.
We are sorry to hear about the struggles you have experienced with entering the license key.
I would love to help you with this topic.
As a first troubleshooting step, could you please provide us with your complete system information in a private reply using the instructions found in the following link?
https://theeventscalendar.com/knowledgebase/sharing-sys-info/
Secondly, can you confirm if you went through all the steps in the following procedure ?
Best regards,
Geoff B.