@Andrew Simpson,Thanks for posting in Q&A.
From your description, I know you want to cancel your windows 365 subscription.
Based on my research, here is a link about how to cancel your subscription in the Microsoft 365 admin center.
However, if the issue still persists, since the limitation of Q&A, it is suggested that you open an online case to get more help.
https://learn.microsoft.com/en-us/mem/get-support
Hope it will help and thanks for your kind understanding.
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