Solicitors must not take unfair advantage of clients or others.
The Solicitor's Handbook provides some examples of breaches of this duty. Paragraph 1.2 of the Code could be breached if, for example, a solicitor communicates with a third party after becoming aware that the third party has instructed another solicitor and obtains an advantage from doing so; or takes advantage of an opposing party's lack of legal knowledge where that party has not instructed a lawyer (for further details on dealing with litigants in person, click here); or knowingly makes a demand in litigation for a remedy which is not something a court could award, e.g. demanding a debtor pay the cost of a letter of claim when it cannot be said at that stage that the cost is legally recoverable.
The duty not to take unfair advantage of others does include the solicitors on the other side, though to a lesser degree. There is no general duty upon one party to litigation or potential litigation to point out the mistakes of another party or their legal advisers (Thompson v Arnold (2007)). Each situation must be judged in the light of its circumstances.
As a solicitor you must however be scrupulously fair. The difference lies in what a solicitor can properly expect an experienced solicitor opponent to be aware of compared with that which a LIP might know. Whether a solicitor has taken unfair advantage of an opponent must be judged on the facts. Where you wish to take advantage of a mistake by the other side, you should look at these body of cases:
Barton v Wright Hassall LLP [2018] UKSC 12
Reynard v Fox [2018] EWHC 443 (Ch)
Woodward and another v Phoenix Healthcare Distribution Ltd [2019] EWCA Civ 985
In Woodward, the Court of Appeal unanimously dismissed an appeal against a decision of Hodge J which concluded that the overriding objective did not require a defendant to alert a claimant to the fact that its service of the claim form was defective before the limitation period expired. This judgment followed the approach adopted by Lord Sumption in Barton v Wright Hassall LLP [2018] UKSC 12: a case with "all but indistinguishable facts". Interestingly, while the defendant owed no duty to warn the claimant, the Court of Appeal suggested that, depending on the facts, the position "may well be different" if a substantial period remains before limitation expires.