To round this off...
I forgot to point out above that there is a hole in the glass itself so I'd worked out that this was where the parts attached.
As I'd had the car sat with the door lining off for a week I decided to bite the bullet and take off the passenger side so I could see how these parts went on:
I can see the plastic part went around the window like a sandwich
I reached around the back and could feel how the slightly curved metal part fitted on (convex or concave), plus there didn't appear to be a nut on the back of this side either, so great I don't have to go hunting for a nut that will fit the bolt.
So this how it fits together:
After a few aborted attempts I worked out that the white plastic part goes on first:
Then you need to gently push on the bar running along the bottom of the glass to give some space between the horizontal rail and the glass, to allow you to slide the large metal guide in place.
The after that, push the bolt on and then the metal part on the rear, ensuring you have it the right way round. Sorry about the poor pics, but hope they help anyone who is ever in the same boat.