I've got a letter from a lawyer that says "Without Prejudice" on the top of it, but the letter sounds really nasty. Why are they telling me they don't mean prejudice but also being a little aggressive.- Anon.
You, my friend are about to be sued. You are also the proud recipient of a Calderbank Offer, so named after Calderbank v Calderbank  3 All ER 333 (EWCA). This is like your letter to Hogwarts, if Hogwarts was a Courtroom. Sometimes the letters say "Without Prejudice", sometimes they say "Without Prejudice Save As To Costs" or some will say something like "This constitutes an offer under Calderbank v Calderbank  3 ALL ER 333 (EWCA), failure to accept this genuine offer by..." Either way, you're dealing with a Calderbank letter.
"Prejudice" in this context doesn't mean what you think it means.
Prejudice in this context is an unnecessarily complicated way of saying "privilege" and not in a white, hetero, middle class way but as in: 'this communication is privileged between you and me and can't be disclosed.'
Calderbank letters, or a Calderbank offer are used between two parties before and during litigation as a way of genuinely trying to settle the dispute between them. Calderbank letters are prejudiced (privileged) between the recipients and can't be tendered into court as evidence. They are intended to be a way for parties to have private (and secret) negotiations that aim for genuine resolution.
Because the Calderbank deliberations are secret, they're not for disputing or admitting facts, that's sneaky and voids their inadmissibility in court.
Why send this passive aggressive sounding secret letter?
Calderbank letters act as a bit of a safety net. They're meant to encourage parties to settle and settle reasonably, courts don't like having you in there any more as you don't like being in them.
Calderbank offers are not meant to be secret forever. If one party has prolonged litigation for longer than necessary and therefore accruing costs because they haven't settled when given the fairest opportunity to do so, then those letters can be tendered into court as evidence of someone's reluctance to bring a case to an end.
The court will decide if one party has genuinely tried to settle and if the other has just pursued a losing case and in doing so extended litigation longer than necessary. The court can then make an order for the party that racked up the other side's costs to pay some or all of them back.
There are certain rules that need to be followed when drafting, replying to, and relying on a Calderbank offer. In fact, it's not unusual in litigation to send two letters at the same time. One a letter of demand that is just plain aggressive and demands you do something; and the other a Calderbank offer which is more passive and less aggressive proposing an offer to settle.
You send two letters because with the former you start legal processes churning, and with the latter you try and settle as soon as reasonably possible so that you can solve your client's legal issue.
You're probably going to get another letter of demand very soon, you should also be getting a lawyer.