Oauth2 Error Renewing Token

Okay I am sure I am missing something simple here we go:

Scenario, using the information here: https://developer.sage.com/api/accounting/guides/authentication/

I am trying to renew an expired authorization token using the following URL:

https://oauth.accounting.sage.com/token?client_id=XXXXX-myClientID-XXXXX&client_secret=XXXX- myClientSecret - XXXX&grant_type=refresh_token&refresh_token=XXXX - myRefreshToken - XXXX

The content-type is set to application/x-www-form-urlencoded

I have a valid trial account for Sage Business Cloud Accounting and using Postman, I can get the original 5 minute Authorization Token and a Refresh Token. I have code written to get the new token using the URL above, however I get the following returned:

{

    "$severity""error",
    "$dataCode""DataParsingError",
    "$message""The data you sent could not be processed.",
    "$source""Proxy"
}
I thought it could be something in the code so I went back to Postman and created a new Post Request and filled in the Parameters and Headers and I receive the same error back. 
What am I not understanding?