Azure: accessing the Usage API using the oAuth key from the Enterprise account

I have an Enterprise account with a 'Usage Api Access Key'. I can use this key to make calls to:

http://ea.azure.com/rest{account-id}/usage-reports

by setting the Authorization header on my REST call.

Unfortunately I can't use these same keys to call the Usage Management API using the an address like the one below with the subscription Id substituted in:

http://management.azure.com/{suscription-id}/providers/Microsoft.Commerce/UsageAggregates?api-version=2015-06-01-preview&reportedStartTime=2015-02-01T09%3A00%3A00&reportedEndTime=2015-02-08T09%3A00%3A00&aggregationGranularity=Daily&showDetails=false

as the result is always: { "error": { "code": "AuthenticationFailed", "message": "Authentication failed." } }

Do I have to create a different type of key for this API?

Any help much appreciated!