Ok, I shouldn’t have suggested it would be a noob mistake, that’s like saying “please don’t reply” 
Solved it eventually by using content type application/x-www-form-urlencoded and putting the url in the body rather than the query string.
I do think though that the error 357 “url: queryParam is required” should not be returned (unless someone can make this work with the url in the query string…??), and the error message I got was somewhat confusing 
Nevertheless, I’m unstuck now!