They need to give you notice before they send you the Bill, by law they must give you the oppurtunity to correct the defective work before they can charge you for it. On the other hand you should be asking for proof that you are responsible for the damage i.e record photographs etc.

You can fight this pretty easily and I would be very suprised if they take it any further once you explain to them that you want to see proof.

I get it everyday at work but I'm usually fighting the other corner. HTH