Car Club Discount Code is :
CARSSS (this gets car + 2 in for £15)
Note : This is supposed to be a exclusive code for car club's but if groupon (see below) gets you in cheaper then fair gaime
Cheaper ticket's at £12 (thanks Greg) particularly for families (£15) are available here (limited time only) :
http://m.groupon.co.uk/deals/events/gl-the-supercar-event/39277486
I've got no idea how they are controlling number's as nobody seems to want to know what clubs are coming and how many attendee's.