Just as an example, this isn't a letter, but a press release so the end out come of what you want is similar. I'm not saying this is the best press release but it did turn the benefit of a magazine exposure on one of my websites into a 5-10 minute segment on BBC radio, including an interview.
http://www.guessmyage.net/gma_press_release_13_09_08.pdf
I also helped co-write another press release, here:
guitar123.co.uk/pressrelease.pdf
(please don't link to this directly)
That got us a meeting with a large magazine publisher, which publishes the majority of guitar related magazines in the UK.
I don't want to criticise you as you've taken the effort to write the letter, just wondering if the way the letter has been written maybe has effected the outcome. Maybe take a look at the two press releases...to some degree they helped get what was wanted, or at least closer to that.