OK, between pouring rain and hail, I managed to get the main event done. I've still got the fiddly bits underneath to put back on, but the hitch and bumper is done - somewhat rough for now, but done!
Here's how it looks.
This was before a welded the brackets on to refit the bumper. I wanted to try and balance how far the bumper stuck out, and how far the receiver stuck out of the bumper.
This is the finished product with the number plate on. It doesn't look quite square in the photo, and I must admit, I didn't check it from afar. Still, nothing a BFH won't fix.
This is the gap behind the number plate. It's big enough to get the pin in and out, but just small enough to hide behind the number plate.
View from above again
and a side view to give an idea of how far it sticks out.