If you don't get a reply from your MP it's very much worth trying to get another 4 or 5 people to write. From campaigning in other areas, I am aware that the threshold of what is "a lot of letters" is in fact 4. If an MP receives 4 or more letters on a particular issue, that's normally enough to wake them up.

BTW - both the MPs I wrote to have replied fairly satisfactorily and both have signed up to EDM331. I posted the infomation to another thread.