89.—(1) In calculating the amount of recoverable excess benefit, the relevant authority shall deduct any amount of council tax benefit which should have been determined to be payable in respect of the whole or part of the overpayment period—
(a)on the basis of the claim as presented to the authority;
(b)on the basis of the claim as it would have appeared had any misrepresentation or non-disclosure been remedied before the decision; or
(c)on the basis of the claim as it would have appeared if any change of circumstances had been notified at the time that change occurred.
(2) In calculating the amount of recoverable excess benefit, the relevant authority may deduct so much of any payment of council tax in respect of the excess benefit period which exceeds the amount, if any, which the claimant was liable to pay for that period under the original erroneous decision.