In this case the Supreme Court was unanimous in dismissing an appeal.
Of CPR 3.9 Lord Neuberger had this to say:
“13. The basis upon which a court should approach an application for relief from sanctions under CPR 3.9 has been authoritatively considered by the Court of Appeal in Mitchell v News Group Newspapers Ltd (Practice Note)  1 WLR 795 and Denton v TH White Ltd (De Laval Ltd, Part 20 defendant)  1 WLR 3926. Although Hildyard J gave his decision refusing relief from sanctions before those two decisions of the Court of Appeal, his reasoning and decision reflected the guidance and approach set out in them. Quite rightly, there has been no suggestion that we should reconsider what was said in those decisions.”
The position is therefore this: Mitchell, as interpreted by Denton, is good law.