A beautifully clear, typed and well-presented letter of complaint is infinitely preferable to a shabby handwritten one.
Make sure any complaint emails or letters are clearly and thoughtfully presented. Typewritten or manuscript correspondence is often taken less seriously (especially if it is illegible!).
For those of you lucky enough to have access to a charming and sympathetic secretary, ask very nicely for his or her help and you may get some useful guidance in achieving a really professional touch to your masterpiece. According to my secretary, a chilled Kit-Kat or a Creme Egg is the going rate for assistance of this kind!