Probem solved by (I think) deleting the same creds on another test app – that was deleted.
(had to undelete the project via the old console https://code.google.com/apis/console , then delete the creds within it, then delete the project again.)
I think possibly creds are valid globally even after an app has been deleted – presumably until the app has been fully deleted (ie undelete is no longer available.)
I’m not 100% sure, but posted this in case anyone has the same issue.