It starts at 10, there is always plenty of parking, entrance I think is a fiver, and if rain looks likely I'd strongly recommend wearing boots.
If you've not been there before, you want the Easterly of two exits from the A303 signed to "Micheldever Station", then pick up a minor road running East parallel to (and along the north side of) the A303 as if you were heading back onto the A303 to Basingstoke. That takes you straight past the airfield gates on your left.
P