iOS 18 Repeating Event from unused email account - cannot delete

Since upgrading to iOS 18 there is now a repeating event that is appearing in my calendar.

This event is from a Gmail Calendar that I haven't used for years; and the account is not (and never has been) synched with the current phone. Clearly Apple have been storing the data of past activity and for some odd reason they are reapplying it to the Calendar.

It will not allow me to delete the event and it is just stuck there as a phantom event.

The Gmail account isn't just not in use; it has been disabled for years. So the issue is not Gmail but Apple retaining data that they shouldn't and not enabling me to delete that data.

Smells a bit of GDPR violation to be honest. However let's assume it's a bug they can fix in time to avoid that.