Hi @Gavin_Preston and welcome to the Forum! 
Sorry for the trouble here. In order to help you with this, I would recommend to contact our Support Team from the email address that was the Billing Owner of the Premium subscription, and they will be more than happy to assist you with this. To contact our Support Team you just need to follow these steps: https://forum.asana.com/t/how-do-i-contact-support/19057/7?u=natalia
Please note our team is currently tackling some backlog, so they’re a little slower than usual getting back to users.
I hope this helps Gavin but please let me know if there is anything else I can help you with.