Yep, the trail will be closed - well, users are asked to avoid it, which is pretty much a polite way of saying the same thing. If you want to spectate just turn up. The main arena is at the top of the estate, over behind the golf course, on the grassy field where the Ashton Court Festival and Kite Festival both take place. I think you can also be a marshall for a day if you want to take a more active role.