18.—(1) The debtor and the creditor may at any time agree in writing as to the events that constitute a default or otherwise give rise to the rights and remedies specified in regulations 19 to 21 and 25.
(2) Where the debtor and the creditor have not so agreed, “default” for the purposes of regulations 19 to 21 and 25 means a default which substantially deprives the creditor of what it is entitled to expect under the agreement.