Unacceptable Assessment

I am currently so upset. I have been a customer of Miway for many years. I recently got into an accident, a small bumper bashing. Was driving in almost still standing traffic. The visible damage on my car was minimal, what I could see was a crack of 5 cm on my bumper. I even checked and I did not see any fluids leaking from my car. So I got in my car and drove on for a while, when I saw that something more might be wrong from the heat gauge. I immediately pulled over and phone them. Unfortunately it was too late and my engine was damaged. I have full insurance as well as Top-up (in worst case scenarios). Now they refuse to write of the car because I drove on - even if it was not long. So they will repair the body and give my car back to me without a working engine. Really disappointing. So now I owe the bank for a car that is no longer derivable.