i clicked the "accept" button as soon as i got the email a few weeks ago, sent the form in the post a few days later, then rechecked the button, clicked again and it said "according to our system you have already accepted". haven't got a confirmation of receipt or anything but to be honest I think it's fine. As long as they're aware of you accepting (whether through post, email, or whatever, i think it's fine.
remember, they need you more than you need them. it is in their best interests to make sure they have enough people.