Thanks for your suggestion Alok. I tried this, but I now have a brand new account, with the relevant email!
So now there are two Cloud accounts linked to the email...
1. Has Maps API instance configured, is working, and is generating charges and invoices (to the email)
2. Is a brand new account, and is linked to the email as a login
At this stage, I think it best to just re-create the API settings, and set up billing in the new account, and switch over the API settings in the App.
That way, usage will transition from unreachable instance to reachable instance.
Still, I find it perplexing that there is no straightforward way to sort this out!