Back the basics.
Reading through the docs again I decided try and and authorise using this url first.
This takes me to a wordpress.com page to ask me to authorise.
So, then it started to make sense to me.
(oauth2-request-authorization "https://public-api.wordpress.com/oauth2/authorize" "2194" "" "" "https://emacstragic.net")
That above function launches your browser on the wordpress OAuth page and asks you to authorise the app to one of your blogs. It also prompts the user in emacs to enter the authorisation code the browser displays.
But selecting my blog and hitting Authrorize gives me an error
You must connect your Jetpack plugin to WordPress.com to use this feature. which is strange because it is already connected.