The best kind of complaining letter, from a practical point of view, is the kind where you write one paragraph, and they have to spend week researching the answer, because otherwise they are worried they could be in real trouble.

Not such fun to write or read of course. Perhaps the thing to do is write the fun one to hgte it out of your system, and remind you of all the details, and then bin it, or post it on the Internet somewhere. And then write the killer one-paragraph one.