Add a new method to a jQuery UI widget

Add a new method to a jQuery UI widget

The technique described in the docs for extending UI widgets works for me when extending an existing method, but not when trying to add a new one.

Rather than repeat the details here, here's my SO question in full:
http://stackoverflow.com/questions/42024327/add-a-new-method-to-a-jquery-ui-widget

Thanks in advance for any ideas.