17.—(1) In this article, the expressions “appropriate amount” and “estimated amount” have the same meanings as in regulation 24.
(2) Subject to article 73, an authority may authorise a contractor to exercise the functions—
(a)of calculating the chargeable amount and the difference between that amount and the estimated amount;
(b)of preparing and serving a notice stating the chargeable amount;
(c)where the chargeable amount exceeds the estimated amount, of requiring the liable person to pay an additional sum equal to the difference, and where the chargeable amount is less than the estimated amount, of repaying any overpayment or crediting any overpayment against any subsequent liability of the liable person to make a payment in respect of council tax;
(d)of determining whether any factor or assumption by reference to which the estimated amount was calculated is shown to be false, and calculating the appropriate amount with a view to adjusting the liable person’s liability in respect of the estimated amount;
(e)of preparing and serving a notice stating the appropriate amount;
(f)where the appropriate amount is greater than the estimated amount, of requiring an interim payment from the liable person, and where the appropriate amount is less than the amount of the estimated amount paid, of making an interim payment to the liable person; and
(g)of requiring that the amount of any overpayment mentioned in sub-paragraph (c) above or the amount which would otherwise be paid to the liable person by way of an interim payment shall, instead of being repaid, be credited against the liable person’s liability to pay council tax in respect of a dwelling other than the dwelling to which the chargeable amount relates.