An order for possession may be set aside by the court if one party fails to attend a trial. [CPR 39.3]
Note: The word “trial” is important. The below rules do not apply to the initial possession hearing as normally only a few minutes are allowed. Therefore if a possession order is granted at the initial “hearing” and the tenant fails to attend, they will normally be allowed to set aside that decision without having to comply with the rules below. [Forcelux Ltd v Binnie  EWCA Civ 854] For the difference between a “hearing” and a “trial” see this article.
Note: if there has been a trial and all parties attended, set aside is not possible. The recourse at that point would be an appeal.
Because there is normally no hearing held for an accelerated possession case, it is possible for a landlord or tenant to set aside the order. This must be made within 14 days [CPR 55.19]
Rule 55.19 Application to set aside or vary
The court may-
(a) on application by a party within 14 days of service of the order; or
(b) of its own initiative,
set aside or vary any order made under rule 55.17 .
After hearing held in relation to a section 8 notice case (or a section 21 case that did not use the accelerated possession procedure)
The rules governing a court to allow a set aside are quite strict on the court.
Rule 39.3 Failure to attend the trial
(1) The court may proceed with a trial in the absence of a party but-
(a) if no party attends the trial, it may strike out the whole of the proceedings;
(b) if the claimant does not attend, it may strike out his claim and any defence to counterclaim; and
(c) if a defendant does not attend, it may strike out his defence or counterclaim (or both).
Where the court strikes out proceedings, or any part of them, under this rule, it may subsequently restore the proceedings, or that part.
Where a party does not attend and the court gives judgment or makes an order against him, the party who failed to attend may apply for the judgment or order to be set aside.
(4) An application under paragraph (2) or paragraph (3) must be supported by evidence.
(5) Where an application is made under paragraph (2) or (3) by a party who failed to attend the trial, the court may grant the application only if the applicant-
acted promptly when he found out that the court had exercised its power to strike out or to enter judgment or make an order against him;
(b) had a good reason for not attending the trial; and
(c) has a reasonable prospect of success at the trial.
“a party who failed to attend the trial” (r.39.3(3))
This must be read in conjunction with Practice Direction, paras 2.1-2.4 (see 39PD.1 ) (cf . RSC O.35, r.1 and CCR O.21).
In Nelson v Clearsprings (Management) Limited  EWCA Civ 1252 , 1 W.L.R. 962 , the Court of Appeal stated that the whole of r.39.5 contemplates a trial in the absence of a party who has been properly served with a claim form in accordance with the relevant rules as to service (that is to say, a party who had formal notice of the proceedings), or in respect of whom service has been dispensed with. Where (1) the party failing to attend trial is a defendant who has not had formal notice (because of a failure to effect proper service on him), and service on him has not been dispensed with, and (2) judgment is given against him in his absence, ... Please login or signup to continue reading this content