Apparently, after someone made the side of the building all pretty with a fake stream and such, it became difficult to reach the outdoor power outlet.
I'm not really sure, but I THINK that's the reason why someone constructed an ornamental bridge going smack into a wall.
However, one morning on my latest visit there, I noticed some Potter fan had come up with a better explanation for the bridge.
If you click on the pic to enlarge it, you'll see the handwritten sign says "Platform 9 3/4."
This made perfect sense to me. :)