We usually park at county hall park and ride which is just a few minutes ride by bus into the city centre. There is a car park near the stadium but it is qued up all day to get in, so I would advise park and ride.If you wish to see the pre meeting entertainment and "soak up" the atmosphere,(the atmosphere starts to build up around mid day in the town let alone the stadium. It really is a brilliant day allround) then you should get to the stadium by 3.30-4.00. The meeting usually gets underway with the parade at 5.00, first race as soon as the parade is over.
Hope that helps
Scottyfan