I have been through the motions creating a test appliction on the HMRC developer portal in order to test updates applied to NAV 2016. I have managed to generate an authentication token successfully in my environment however, every time I try to do anything such as submit a vat return or retrieve vat liabilities I get the error message:
The client and/or agent is not authorised.
Answers
https://developer.service.hmrc.gov.uk/api-documentation/docs/api/service/api-platform-test-user/1.0
This can be done on-the-fly using an application such as Postman.