Hi @Andy Saxton ,
Something in the following links could be of help:
How To: Manage inactive user accounts in Azure AD
https://learn.microsoft.com/en-us/azure/active-directory/reports-monitoring/howto-manage-inactive-user-accounts
How to: List inactive users in Office 365
https://community.spiceworks.com/how_to/104316-list-inactive-users-in-office-365
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Best regards,
Leon