I have discovered that the skew bridge in my photo above is even more odd. Only half is a real skew arch, with the characteristic helical brick courses in the roof (the engineering behind a skew arch makes my brain melt, but let's just say that a conventional arch would fall down). The other half of the brick has conventional bricklaying. The portals at each end aren't parallel!
This has been done in order to accommodate a road junction above and I suspect the skew part was added years after the original, since the houses above are different ages.