Invalid client identifier when trying to authenticate for Application-restricted RESTful APIs sandbox environments

Today when trying to continue development using the sandbox environments, when I go to authenticate, I am getting a 500 error: Invalid client identifier.

Request URL: https://sandbox.api.service.nhs.uk/oauth2/token
Method: POST
Response Content: “{“fault”:{“faultstring”:“Invalid client identifier {0}”,“detail”:{“errorcode”:“oauth.v2.InvalidClientIdentifier”}}}”