Good morning / good afternoon @neilegunning and thank you for posting!
In case there are other tenant admins who are still able to log in, I would like to offer a workaround described in the following doc - How to Update Tenant Member Email
But if you are a sole tenant admin, or all admins relied on emails/email domains that are currently deactivated, please let me know and I will start our established process of bringing back the lost access.