Having experienced this too many times, I was being ultra-paranoid yesterday. As the email was kinda important, I was hitting the "Save as Draft" button every few minutes. Did this several times, so I had a really up-to-date backup of the mail when I hit Send. But it seems KCL's dear webmail program saves its drafts somewhere internal, not in the Drafts folder used by the "real" email program that I use when I'm at college - and not only that, it eats the draft when you hit send. Even if the message doesn't actually get sent!
Fucking piece of crap software. In future, I'm going to write my email in Microsoft sodding Word. At least when that dies, you can usually recover your work from somewhere.
I suppose it's good preparation for my night of installing The Sims, though...