Sorry for the bump but here's some ideas (that have worked for me) I thought might be worth sharing.
Produce a demotape of sketches and send it out - many people will be more ready to listen to something than actually sit down and read through pages of sitcom.
Watch the credits of programmes you like and note down the names of the production assistants and other people involved -- the key is to find people who are in a position of power but not necessarily well known.
If you're under 18 or a student be sure to mention so when you contact them; a lot of the time producers will pity you and give detailed feedback or invite you in for tea!
In addition to the internet, there's other ways to find producers and writers - try attending BBC Events (the ones at the Fringe are brilliant for networking) and get friendly with the people there. Twitter is also good for networking, I've made some producers laugh and had them follow me -- you can sometimes get a discussion going privately after this.
Also (this one's a bit sneaky) you can often guess email addresses of people using the formats
john.smith@somethingproductions.co.uk
Or
jsmith@somethingproductions.co.uk
This works for a lot of places, but it's not always reliable (and it's a bit rude, but I've found many producers are cool about it if you're polite.)
Finally, don't send ANYTHING until you're absolutely positive it's good. I've lost contacts because I've (stupidly) failed to impress them the first time and they've never given me another chance. When you're confident you're good enough, get your stuff it in the hands of the right people but never before.
Hope some of this helps (sorry for the spiel.)