Sometimes I need the obvious pointing out to me! Huge thanks. Next thanks go to the Discourse team - I had not previously found the /logs page in the web interface - amazingly valuable and easier than digging around on the server.
Still not fixed, but at least I am not running blind any more!
The logs suggest I am not correctly submitting the formdata correctly.. This is response with either the json or plain formatted version:
Discourse::InvalidParameters (user_id or username must be present)
/var/www/discourse/app/controllers/groups_controller.rb:229:in `remove_member'
/var/www/discourse/app/controllers/groups_controller.rb:229:in `remove_member'
/var/www/discourse/vendor/bundle/ruby/2.3.0/gems/actionpack-4.2.9/lib/action_controller/metal/implicit_render.rb:4:in `send_action'
/var/www/discourse/vendor/bundle/ruby/2.3.0/gems/actionpack-4.2.9/lib/abstract_controller/base.rb:198:in `process_action'
/var/www/discourse/vendor/bundle/ruby/2.3.0/gems/actionpack-4.2.9/lib/action_controller/metal/rendering.rb:10:in `process_action'
I am not Ruby literate - but one weird thing is that the line 229 in that source file is not in the ‘remove_member’ function - it is in ‘add_members’ function.
As I understand it, the data user_id for this method should be added as form data - that is what the actual discourse app seems to do, but so far I cannot replicate correctly in Go.