If your MP things thats offensive.....I suggest you string him along a bit more
Isn't there a rule where they have to reply to letters?
I'd write one back, dissecting his points one by one, but keep it professional. Point out that if you voted for him or not is irrelevant, he is still representing you.
Oh and then go to the papers. Even if they ain't interested, you've lost nothing.
Its that exact kind of attitude that has got Labour where it is.