I hope you are not expecting a "Wave" of response!!!
As with any letter, the secret is to be concise, to the point and stick to the facts. If it is the first letter then outline the problems, what you have done so far to resolve them and what you hope the company will do to assist you. Be polite but firm, as the letter may well be the first time someone in authority becomes aware that you have a problem - dont upset him....just yet!
If you get nowhere with this approach, then time for another letter, referencing the fist and stating what you demand to be done within a given time limit (be reasonable). State that unles this happens you will take alternative action etc.
If this gets you nowhere then the final letter is the shortest and simplest - just say that "despite my letters of so and so date you have failed to address my problems to my satisfaction. You therefore leave me no alterantive but to take (whatever) action without further recourse to you"
mention the fact that they may incur further costs ect and see what happens. But initially, be polite, dont sound off, and stick to the facts!