The answer is found in CPR Part 27.11. I suggest you read CPR part 27.
(1) A party �
(a) who was neither present nor represented at the hearing of the claim; and
(b) who has not given written notice to the court under rule 27.9(1),
may apply for an order that a judgment under this Part shall be set aside and the claim re-heard.
(2) A party who applies for an order setting aside a judgment under this rule must make the application not more than 14 days after the day on which notice of the judgment was served on him.
(3) The court may grant an application under paragraph (2) only if the applicant �
(a) had a good reason for not attending or being represented at the hearing or giving written notice to the court under rule 27.9(1); and
(b) has a reasonable prospect of success at the hearing.
(4) If a judgment is set aside�
(a) the court must fix a new hearing for the claim; and
(b) the hearing may take place immediately after the hearing of the application to set the judgment aside and may be dealt with by the judge who set aside(GL) the judgment.