In the case of any transaction it shall be assumed -
(a) that a charge payable at a time which cannot be ascertained at the date of the making of the agreement shall be payable on the relevant date or, where it may reasonably be expected that a debtor will not make payment on that date, on the earliest date at which it may reasonably be expected that he will make payment; or
(b) where more than one payment of a charge of the same description falls to be made at times which cannot be ascertained at the date of the making of the agreement, that the first such payment will be payable on the relevant date (or, where it may reasonably be expected that a debtor will not make payment on that date, at the earliest date on which it may reasonably be expected that he will make payment), that the last such payment will be payable at the end of the period for which credit is provided and that all other such payments (if any) will be payable at equal intervals between such times,
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