Please be aware that it is a little 'snarky' in places. I could clean it up but then it would be far too boring to read (or write). (Google "Miss Snark" for the origin of the word).
Feel free to disagree - I may even amend it if you can persuade me to - but this isn't about writing the best covering letter in the world; it's about NOT writing one that'll get your submission rejected before they've read the all important sample chapters.