Progress…maybe?
Removing convert to JSON from object block for the Header stopped me getting the Auth error. So, I thought maybe if I also do that for the body that might work… I get something new:
422{“detail”:[{“loc”:[“body”,“url”].“msg”:“str type expected”,“type”:“type_error.str”}]}
So, why does it think my url is not formatted as a string? 
But, if I do something daft e.g. leave the url as ‘url=res.cloudinary.com…etc’ then it recognises that the url isn’t correct and I get:
'200{“error”:“There was an error uploading the file. Check to make sure your file or file url is valid. Also, make sure that you uploaded a valid file type, language, and task.Note: if you set diarization to true, fewer files types will be accepted!”}
ofc - I haven’t set any of those additional parameters so they should all be default and working OK. It certainly worked when I ran it in POSTMAN.