People become a lot more critical of their vehicle after an accident. This is primarily because you're paying more attention. Plastic welding a bumper is an appropriate means of repairing the bumper, but you should certainly not be able to tell. Bearing in mind you have damaged the rear bumper, the silencer on your car sits tucked up behind it. It's very possible it has been pushed back, and kinked somewhere. You need to get it on a ramp, get underneath with the engine running,and listen for any leaks or blowing (look for black soot), and look at every bend right up to the engine down pipe and manifold. Find a friendly garage or Kwik-Fit and then photograph what you see. Do not repair it at this point unless it is unsafe. Then recontact your insurer and send them this evidence.