Request for clearing an overdue payment

Please help me improve the following draft letter to a tenderer, who owes our department an electricity bill charges. Thank you very much.

Dear Mr. Dowell,

We refer to our department's account record on your car park business under mutual tender agreement, we are surprised to see that there has been an electricity bill overdue amounting to $1,000 since the commencement of the tender agreement, namely 01.11.2013. As a law-abiding tenderer and to ensure the accuracy of public financial management, we should be grateful if your company can pay off the above overdue, regardless the interest payment incurred by the end of this month. Otherwise, we may reserve the rights to lodge a lawful accusation against your payment failure, depending on the situation.

Yours sincerely,
HH