What makes a letter work
Not the tone. A letter carries weight when it names the right legal ground, states the facts that ground actually needs, makes a specific demand with a real deadline, and stops. Most template letters fail on the second of those: they are written to sound serious rather than to satisfy a test, so the recipient’s solicitor reads them and sees nothing they have to answer.
The ground decides what the letter may demand
Each of these letters is a different legal argument, and the rules are not interchangeable. A harassment demand needs a course of conduct — conduct on at least two occasions, under s.7 of the Protection from Harassment Act 1997 — so a letter describing one incident is asking for a remedy the statute does not provide for it. A defamation demand needs serious harm under s.1 of the Defamation Act 2013, which for a company trading for profit means serious financial loss, and it has to be brought inside a year. An invoice dispute is strongest when you pay the undisputed part and withhold the rest, because that is the conduct a court expects to see.
And some letters carry a risk in the other direction. A demand that threatens infringement proceedings over a registered trade mark can itself be actionable by anyone aggrieved by the threat, under s.21A of the Trade Marks Act 1994 — which is why the cease and desist tool will not assert a registered mark unless you give it a registration number.
Sending one and answering one are different documents
Four of the thirteen exist because a letter landed on your desk rather than the other way round: answering a cease and desist, a solicitor’s letter, an employee grievance, a disputed invoice. Those are not the outbound letter with the names swapped. Answering well means conceding what is true, disputing what is not, and admitting nothing you did not have to — and where the claim against you is weak, saying so plainly and flagging any threat that was not justified.
Jurisdiction is a question the letter has to answer
England & Wales, Scotland and Northern Ireland part company on most of this, and the difference is often the whole point. Defamation runs on the Defamation Act 2013 in England and Wales, the Defamation and Malicious Publication (Scotland) Act 2021 in Scotland, and in Northern Ireland on a 2022 Act that was passed without a serious-harm threshold, so the bar there is lower. Harassment in Scotland runs through s.8 of the 1997 Act, with interdict and a non-harassment order rather than the English injunction. Every letter here asks where it happened before it decides what you may demand.