Unable to delete an icloud email address

We have an old @icloud.com email address (IMAP) linked to our account and we cannot delete it. It's an alias for our Apple ID & icloud Mail and the instructions simply say to delete it but won't allow it.

When managing the Apple ID section 'where you can be contacted' that @icloud.com email has a ? next to it with the hover text telling me it's an alias for the Apple ID. Can't delete or disable it there or in iCloud Mail.

We also get a redirection to iMessage and Facetime settings but nothing we do will get rid of it.

Anyone have any suggestions??

Cheers 😎