Ian,
All good to turn up whenever you want through the day, and taking photos from the sidelines is fine. We have the photographers that can move about the place, and get closer shots; with their gear, we offer the service for people who want professional-type photos of their cars; they're frigging good at what they do I don't think you'd annoy anyone taking your own pics from the sides.
If you want to actually be a passenger in a car, you'll need ankle to wrist clothing, a helmet, and sign a short waiver that says you understand the risks of motorsports. Having said that though... It's a wide open pad with no trees or banks or anything