In my limited experience (I've only voted in two constituencies during my life) MPs are actually pretty good about responding to comms. Now the cynic in me says it probably isn't them, but I've had an email back and forth with my current MP (Nick Brown, Newcastle East) during the past year, so it can't just be canned responses.

Whether you every actually influence an MP's position, or compel them to action on the basis of such a letter is a different matter entirely however.