Here's a beware.

I did the fax-your-mp thing last week and sent it off with no problem. Spent some time doing a nice letter to my (con) MP. After 10 minutes, I got a message back saying that the fax couldn't be sent. No way I could get the original info back to send any other way. Best part of an hour wasted.

Moral: copy the stuff you've done somewhere before you press the [SEND] button! :-(
Ian