Hey Jack, thanks for that. The link helped me search for this modal with the token status which clearly shows the issue, there is no refresh token being granted/received/stored!
I believe I've solved the problem by adding access_type=offline to the end of the Auth URL and changing the prompt method to "Consent". I now see a refresh token - I'll let you know if it doesn't work 
It would be good to include this info in the OAuth setup docs if possible!