Exactly the same thing happening with us and we also use Fasthosts. Settings deactivated again today. Have to go in, reactivate and ok for a while again. No problems at all for us until a couple of weeks ago. Fasthosts have made changes to their email recently (e.g. they now have spam filtering) so it is probably something that has crept in with the changes.
@datapartners, please post if you find anything out. We will investigate further in the new year.
@QFMathew, what is quickest way to tell if email has been successfully sent? In your message you say the email is being successfully sent. How can one tell?