However, on the Node Access User Reference's project page, it shows a screenshot of a configuration form which I cannot find. I want to grant referenced users editing rights, but cannot find where to do this. Where do I find this form?
I found a clue in the README.txt which I'd previously missed
Create a field to reference users in a content type using the Field UI. The field's configuration page will contain the settings for Node access user reference.
I edited the User Reference field I'd created, and on the Edit tab was a collapsed heading for Node access user reference that I'd previously missed. Clicking the heading revealed the configuration form.