The category does get added and the upload works, regardless of the errors. However, clicking Generate embed on the uploaded file results in:-

PHP ERROR [1] Class 'Media_renderer_with_fallback' not found in sources\hooks\systems\media_rendering\audio_general.php on line 25

No idea why it needs audio general for a zip, but then I generally have no idea about most things :)

"No idea why it needs audio general for a zip, but then I generally have no idea about most things" - the media rendering system is really sophisticated and it scans through various handlers trying to find what can handle it. Was a dependency issue in the code though.