If you go on the Silverstone website and look under how to get there for the grand prix, there is an option for buses.. It did seem to be very well used. We actually got stuck in traffic on the way back on the Sunday. Journey is normally around 40 minutes, but it actually took about an hour and a half to get back to Milton Keynes. Going, on Saturday and the Sunday, was a breeze though, and also coming out on the Saturday, once we'd cleared the first few roads leading from the circuit. I'd definitely use it again if I went another time.