Hello Graham (another Graham here!)
I see that we have both posted queries on Moodle about the Moodle APP. Is it possible that you are experiencing a similar issue to me--only those with Administrator privileges can log on to the APP? Anyone else gets a quirky/geeky, unhelpful message telling them that a parameter is missing.
I have spent hours going through Moodle Docs and the Internet to try to find a solution. I have exhausted all the obvious ones like allowing mobile web access. I do see, however, that when I try to enrol anyone who is not an Administrator for the APP, I get messages back indicating that the other people have missing capabilities. Again not very helpful because there is no advice on how to give everyone those capabilities.
My intention is to allow ALL authenticated users to also access the Moodle website through the APP. I am really surprised that it is proving to be so difficult to achieve...