It turns out that it was a user privilege issue. Kudos to James Tinmouth for assisting in resolution. I was using a new management user I'd setup with read-only privileges as I only ever intended to use monitoring commands such as "show aps" and "show clients". It turns out it has to be a management user with all privileges. Personally this feels wrong, especially as the curl command will be sent in clear text!
@Aruba - can I suggest a few options to make this better? I would log an Innovation ticket but currently don't have permissions!
1. Reuse the Central API syntax so it can be run against the Instant VC. That would be awesome!
2. Enhance the existing implementation of monitoring calls in 8.6+ so you can pull result sets for all common monitoring type queries such APs, clients and others.
Thanks
Chris