A little update on the module.
I've built the bridge, using some plastruct angles, sheet and Wills "Vari Girder" sections, and given it a coat of paint.
I measured it as needing 15 panels each side to get from one side of the bridge to the other. Although I originally scaled the bridge size roughly from Google Maps, and used a guess based on road vehicles for where the lanes should be, I was a little surprised to go back to the real bridge image and count that it was in fact 15 panels wide. So I'm fairly accurate in that respect.
I also took another street view image showing the underside of the bridge which I worked on approximately for the cross members. I couldn't measure them or the spacing so I went for what looked right, and once the first one had been done at the correct angle, I simply used three 5p coins to ensure the next was parallel, and continued across. Really there should be more, but I had a limited amount of angle left and given that most people either won't think to look, or won't know, it's good enough for me.
Using the first road vehicle I could find, I tried to take a 'road level' view of a similar angle and I don't think it has come out too bad so far, do you?
Next I need to work on the concrete panels, which I've downloaded and printed out from Model Railway Scenery, then I can attach the bridge sides permanently to the spacers and begin work on the scenic side of things.