Nick,
Took a walk up to Silly Bridge yesterday, from what we could see there is no additional arch in the extended pillar. The bridge from wall to wall was 15'9" wide. There are large stone parapets now so it was not possible to gauge wall thickness. Hope this helps with your research.
Also, at the line side towards Hutts yard was one of their old coal wagons, identical to the one pictured in the book 'Wallingford Branch'. It was too far away to get a shot but I will be asking if I can get access to photograph it. I actually have two Robbies Rolling Stock Hutts wagons so it was good to see one for real. I live in the road adjacent to the main line, back in the day it was just a farm track. This summer we were fortunate enough to see a bit of borrowed steam on the preserved line.
Best wishes,
Dean.