Back when I was doing more corporatish law, I would often close my letters by thanking them in advance for complying with my demands and noting that I would file a lawsuit in 10 days if they didn't comply.
You may want to add that at the end of your letter so that they understand there will be consequences if they don't immediately repair the flawed relationship.
I can't say that this has ever actually repaired any relationship or really did anything positive for the letter recipient, so it may not actually be a good idea in your particular situation.
Sometimes it makes the recipient even angrier then they were before.
In fact, on sober second thought, I recommend *not* threatening to sue them.
Hope this helps!
