Better than buy one get one free! Full price is £36 for adults!
Valid from 28/3 to 19/7 excluding 6-10/7 & 13-17/7.
Just print out the voucher and hand it in when you pay. EDIT - voucher says you need the magazine. Not sure if they'll ask for it, might be worth risking? UPDATE - I used this yesterday and they didn't ask for the magazine. The woman scanned the barcode and it said £16 so it was a pound cheaper!
Anyone looking for free parking, there's a road just past the entrance called Wooton Lane, you can park on there for free! It's a 5 minute walk to the gate. You can see it on google maps if you search for postcode: ST10 4DB