I would agree that a letter is the way to go. Write down clearly what you want from them and detail the non-returned calls and bad service that you have been getting etc.
I was in dispute with a travel agent recently and my phonecalls and emails were getting fobbed off for 6 months. I then wrote a strong letter and had a cheque in the post to me within a week (compensation for an error they made, a long story)
Its very hard for someone to ignore a letter or fax compared to an email. Once something is on paper and goes into someones in-tray it has to be dealt with.
Best of luck.