This is a subtle hardening of the rules. The 2011 rules said you must not put forward facts or arguments that you do not consider to be properly arguable'. The current rules have made it a positive obligation only to make assertions or put forward statements, representations or submissions to the court or others which are properly arguable'. The Code does not define the term properly arguable', but to demonstrate compliance you should ensure you have a note on your file in which you record the contemporaneous evidence you have and make it clear why a proposition is properly arguable. This could be part of an opinion on the merits or an advice, but if none, then it needs to be recorded in a file note. After all, if you do not know certain information at the time that later comes out and renders your argument unarguable, how else will you be able to prove that your point was arguable at the time you argued it. For this reason, your file or file note should record what you have seen and anything you know to exist and be relevant, but which you have not seen.
Be that as it may, it still leaves the way clear for all lawyers on the losing side of a strike out action to be guilty of professional misconduct. That can't be right of course, but there seems little other possible interpretation. It therefore puts even more importance on the file notes that you keep. You will need to make sure that your file makes it clear what you knew and why you took the view that prospects were at least 50%. The rules do not say that the properly arguable' test is subjective, but if it were to be objective, then it would be impossible to comply and that simply cannot have been the draftsman's intention.
It is worth noting that the duty under paragraph 2.4 is similar to the previous duty imposed by Rule 11 of the 2007 Code. Rule 11 included a provision that "a solicitor could not draft documents containing any allegation that he or she did not consider properly arguable." Rule 11 was considered by the Court of Appeal in Richard Buxton (Solicitors) v Mills-Owens (2010) in which it was held that solicitors could lawfully determine their retainer when required by a client to advance an argument which they considered to be untenable. An abbreviated version of the 2011 rule is now included in the current Code. This does not therefore help with the assessment of what is properly arguable, but it does provide authority that if the client wants you to argue something which you don't think is arguable, then you can (and must, as it creates an own conflict) withdraw, even if in the middle of a case. Our Engagement Letters contain a warning to this effect.