BGT don't send out letter telling you when you're going to be on as it's edited quickly - and it's a huge production. They don't have enough time to send letters out to all the people in the show. They may, however, send the person whom entered ThinkBikes an email saying that the series has started and to expect to be seen between weeks "x" and "x". It's what they do for Xfactor, and it's the same production team, so I'm guessing they're gonna do it the same way.