National Rail is offering 2-for-1 on Thorpe Park tickets when purchasing a National rail ticket with Staines as the final destination for the day of your visit in 2014.
To obtain this offer you must register then download and print the 2-for-1 voucher and show your National Rail ticket at the Thorpe Park ticket booth on arrival.
It may be worth doing this even if you are not travelling by train. A cheap single ticket is usually available for a couple of pounds (e.g. to the next station on the line).