Daniel, I have a question about writing a letter to the opposing party’s lawyer. The person I am chasing for many has

Any correspondence with the clear intention of negotiating is generally assumed to be Without Prejudice. That said, it certainly doesn't hurt to write Without Prejudice on the top to make it clear that such shall not be used against you in any future legal proceedings, even if it's not exactly an offer.

If you attach evidence to the Without Prejudice letter, although you could argue that evidence is also without prejudice, they may still try to use that evidence at any trial if it appears as though it helps them, and then it would be up to you to object by arguing it was all Without Prejudice, and then up to the judge to rule on the issue.

Does that answer your question?

I will clarify. Generally, any negotiation correspondence is assumed to be without prejudice, even if you don't write it. For other correspondence that is not necessarily negotiation correspondence, will be assumed with prejudice, so to avoid that you may wish to write Without Prejudice on it to gain that protection.

Again though, still be careful as even though writing Without Prejudice on it would let you argue that it can't be used in court, they may still try to use that evidence at any trial if it appears as though it helps them, and then it would be up to you to object by arguing it was all Without Prejudice, and then up to the judge to rule on the issue.

Does that answer your question?

You're welcome, all the best!

I actually just found this which says what I was saying, just more succinctly:

Without prejudice. The without prejudice (WP) rule will generally prevent statements made in a genuine attempt to settle an existing dispute, whether made in writing or orally, from being put before the court as evidence of admissions against the interest of the party which made them.

My information above still stands, this just articulates my points better I feel.