I believe Friday is usually the quietest day but more expensive to get in. Take plenty of money for the rip off car park charges, as Mike says they are big, it can be amusing watching people trying to remember where they parked, until you realise you are in the same boat.
I have heard after 6pm the car park staff go so you get out for free, never had the nerve to try it as knowing my luck I’d end up getting locked in.
Been for the last 2 years after a 20 year gap and it is certainly a lot bigger than it used to be, you will do a lot of walking. Well worth a visit especially last year when they had a type 55 Bugatti although we the great unwashed were kept a reverential distance, you can get close to a lot of very nice cars, especially the GT40's.