However this has led me to a problem - obviously to test the basic functionality (e.g. getting my account balance) I need to put in my secure banking details
You're doing end-to-end testing and unit testing at the same time it sounds.
Don't do that. Distribute your unit tests. They show patterns of use of the API you are writing.
Keep your end-to-end tests closest to your QA group (yourself likely). They are more brittle, unlike unit tests. Unit tests are small, discrete. end-to-end layers your unit tests in ways that can break when integrated, lack a perfect environment or are configured in odd ways.
Persons w/ a large POV will value them more because they are more likely to find edge case bugs.