So I know I said the next step was to form the roof, but I managed to forget to take any photos of that stage and now it's attached to the body it's difficult to show you anything interesting. I can tell you that the curve of the roof was formed using the scalpel blade handle and rolling the roof between the handle and my naked thigh, I found the trousers were getting in the way! So with that horrid image implanted in everyone’s mind let's move onto painting.
I'm guessing that the floor of the original van was probably just the bare metal, but as the 3D print had ridges (a side effect of the printing process) running across the width of the floor I thought I'd paint it as a well worn wooden surface using the second approach I tried recently. This seems to have worked really well and I finished off the painting with dirty brown upholstery on the benches. Most of this will be invisible when in use but I think it looks pretty good.