Ok yeah, it looks like I was using the incorrect user_id value.
FWIW, my user account is both the organisation owner and the user who created the Extension.
I was extracting the user_id value from the Configuration Service screen of the Developer Rig.
Initially I was using the Use Developer Rig user value, which didn’t work. But when I use the Use current user value it works fine.
So presumably the user_id returned by User current user is my user ID, correct?
So then where does the user_id value for Use Developer Rig come from?