The Court of Appeal has recently considered the test for allowing a “late” amendment (as opposed to a “very late” amendment which puts a trial date at risk) in CNM Estates (Tolworth Tower) Limited v Simon Peter Carvill-Biggs Freddy Khalastchi  EWCA Civ 480. Mark Simpson KC appeared for the Defendants (leading Isabel Barter).
The Court of Appeal held that it is a necessary condition that the claim as amended should have a real prospect of success. This is the same test as applies on a summary judgment application, Elite Property Holdings Ltd v Barclays Bank Plc  EWCA Civ 204 at  and  per Asplin LJ. However, even if an amendment has a real prospect of success, whether to allow the amendment remains a matter of discretion.
The Court of Appeal held that, aside from very late amendments, the perceived strength of the case is not normally a factor to be taken into account when exercising that discretion (per Sir Geoffrey Vos MR and Newey LJ at  and Males LJ at ). It would never be appropriate to conduct a mini trial, Okpabi v Royal Dutch Shell Plc  UKSC 3,  1 WLR 1294.
The Court of Appeal (by a majority) indicated that it would have allowed the appeal, but the claim was compromised shortly before the judgment was handed down.
Judgment can be found here.