The debtor under a regulated consumer credit agreement shall not be obliged to pay interest on sums which, in breach of the agreement, are unpaid by him at a rate-
(a) where the total charge for credit includes an item in respect of interest, exceeding the rate of that interest, or
(b) in any other case, exceeding what would be the rate of the total charge for credit if any items included in the total charge for credit by virtue of rules made by the FCA under paragraph (2)(d) of article 60M of the Regulated Activities Order were disregarded.