If your going to Thorpe Park either today (30/07) or tomorrow (31/07) then buy your tickets direct from them http://www.thorpepark.com/prices/tickets.aspx it's 2-4-1.
However if you'd like a little more flexibility (until 07/11/10) the go to cokezone.
http://www.cokezone.co.uk/home/catalogue/reward/prod950062/Entertainment/THORPE+PARK+2+for+1 and register for 2 points, answer one click question for 2 more points and answer another little thing for 5 more points! boom!
9 points free to spend willy nilly. I've entered the competition to win 4 tickets and some food vouchers for thorpe park and also purchased the 2-4-1 voucher.
Still got 2 points, probably will forever now.