CPR 39.3 is in the following terms:

Failure to attend the trial
39.3(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(GL) 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).
(2) Where the court strikes out proceedings, or any part of them, under this rule, it may subsequently restore the proceedings, or that part.
(3) 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.(GL)
(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—
(a) acted promptly when he found out that the court had exercised its power to strike out(GL) 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.

In TBO, Lord Dyson M.R cited from CPR 39.3 as to the three conditions to be met and then said,

10. It was common ground that the third of these conditions was satisfied. But it was in issue whether the first and second conditions were met. The judge held that the defendant (i) did not have a good reason for not attending the trial on 30 June and (ii) had not acted promptly once it had found out about his decision of that date. The defendant says that the judge was wrong on both points.

The Master of the Rolls then cited extensively from Bank of Scotland Plc v Pereira [2011] 1 WLR 2391 where Lord Neuberger MR had said,

24. …An application to set aside judgment given in the applicant’s absence is now subject to clear rules…the court no longer has a broad discretion whether to grant such an application: all three of the conditions listed in CPR 39.3(5) must be satisfied before it can be invoked to enable the court to set aside an order. So, if the application is not made promptly, or if the applicant had no good reason for being absent from the original hearing, or if the applicant would have no substantive case at a retrial, the application to set aside must be refused.
25. On the other hand, if each of those three hurdles is crossed, it seems to me that it would be a very exceptional case where the court did not set aside the order. It is a fundamental principle of any civilised legal system, enshrined in the common law and in article 6 of the Convention for the protection of human rights and fundamental freedoms that all parties in a case are entitled to the opportunity to have their case dealt with at a hearing at which they or their representatives are present and are heard. If the case is disposed of in the absence of a party, and the party (i) has not attended for good reasons, (ii) has an arguable case on the merits, and (iii) has applied to set aside promptly, it would require very unusual circumstances indeed before the court would not set aside the order.
26. The strictness of this trio of hurdles is plain, but the rigour of the rule is modified by three factors. First, what constitutes promptness and what constitutes a good reason for not attending is, in each case, very fact-sensitive, and the court should, at least in many cases, not be very rigorous when considering the applicant’s conduct; similarly, the court should not pre-judge the applicant’s case, particularly where there is an issue of fact, when considering the third hurdle. Secondly, like all other rules, CPR 39.3 is subject to the overriding objective, and must be applied in that light…

Lord Dyson made the following points of interest,

26…there is a material distinction between an application under rule 39.3(3) and an application for an adjournment of a trial. If the court refuses an adjournment, there will usually be a trial and a decision on the merits, although the unsuccessful applicant will be at a disadvantage, possibly a huge disadvantage, by reason of the absence of the witness or the party himself. Despite their absence and depending on the circumstances, it may still be possible for the disadvantaged claimant to prove the claim or the disadvantaged defendant to resist it. I accept that, in some cases, the refusal of an adjournment will almost inevitably lead to the unsuccessful applicant losing at trial. That is a factor that must be borne in mind when the court exercises its discretion in deciding whether or not to grant an adjournment. But if the application to set aside a judgment under rule 39.3(3) fails, the applicant will have had no opportunity whatsoever to have an adjudication by the court on the merits. This difference between an application under rule 39.3(3) and an application for an adjournment of the trial is important. Although it has not been articulated as the justification for generally adopting a more draconian approach to an application for an adjournment than to an application under rule 39.3(5) , in my view it does justify such a distinction…
37 It is not in dispute that, if an applicant succeeds in crossing the three hurdles set out in rule 39.3(5) , he must go on to persuade the court to exercise its discretion to set aside the order. As Lord Neuberger said in Pereira at para 25, if each of the hurdles is surmounted, it “would be a very exceptional case where the court did not aside the order”. In my view, there is nothing exceptional about this case to justify depriving the defendant of the opportunity of defending this substantial claim in circumstances where it has satisfied all three of the conditions prescribed by the rule for setting aside the order.