Did the MPs you contact at least get back to you, even with a form letter toeing the party line? I wrote to my MP and tried to contact him through Twitter and, at the time, made my letter public. I never got a response.
When your job is "representing" a group of people, you'll take notice when a pile of those people disagree with what you are doing.
We can't break the camel's back if we don't add straw.
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.
Generic Response. I binned it and decided never to waste my time again.
Altogether a demoralising experience.