Most likely booked in advance i'd imagine. My company did a day out there 2 weeks ago (sadly i coudlnt go) but from what I hear it was isolated for them all day, so any public rocking up would have gone away disappointed.
For a passenger experience you really need to book in advance. However at weekends and school holidays we do Hot Laps and Mud Laps. These are bookable on the day only and will be 4/5 laps around the track. £15 a person and there will be 3 spaces in each car.