My back box snapped off today too. Luckily I had just turned in street, outside my flat. I was able to crawl the 50 yards to my garage, where I removed the box. I then drove to my local Kwik Fit, another 300 yards. They checked for any other damage (all okay), ordered a new box and I left the car with them. An hour later they phoned to say it was ready.
£138.25 lighter, I drove home.
I could have bought a box much cheaper, but then I would have had to get it fitted, and as I need car tomorrow doing it myself was not an option.
The box was not rusted. It was a clean break. Obviously, the garage that replaced the rest of the exhaust last spring, had over-tightened the joint, allowing no slack. Fatigue had caused it to fracture.