I say Sue the ass off of the Dept of Transport , it must of been some crater thats all i can say!!! Id of taken pics and stuff....however we normally shrug it off and move on...im sure this guy doesnt give a toss , prob luves the plug for his Company.
No question, if any of us are ever in a similar situation, the local authority will pay up. You may need to fight them, but if it was not clearly marked they have to pay for the damage.