Bank validation API

These API endpoints validate UK sort codes and account numbers, and provide bank and branch details.

bank	Accepts a sort code and account number and returns a `true` or `false` to indicate whether it is valid and provides details about the bank and branch.
bank/sortcode	Accepts a sort code and returns `true` or `false` to indicate whether it is valid and provides details about the bank and branch.
bank/iban	Accepts an IBAN and returns `true` or `false` to indicate whether it is valid and, for UK IBANs, it also provides details about the bank and branch.