18.—(1) If it appears from the certificate of service of a default or fixed date summons that the summons has been delivered to a person under rule 10(1)(b) but it is doubtful whether the court will be satisfied that the summons has come to the defendant's knowledge in sufficient time, the proper officer of the court for the district in which the summons is to be served shall give to the plaintiff notice of doubtful service.
(2) Where such a notice has been given and the defendant does not deliver a defence, admission or counterclaim or, in the case of a fixed date summons, does not appear on the return day, the plaintiff may be required to satisfy the court that the summons has come to the defendant's knowledge in sufficient time.
(3) In this rule “sufficient time” means
(a)in the case of a default summons, sufficient time for the defendant to deliver a defence, admission or counterclaim within 14 days after delivery of the summons under rule 10(1)(b), and
(b)in the case of a fixed date summons, sufficient time for him to attend on the return day.