I couldn't see this posted already:-
I'm too old :( but a good deal if you were born in 1970! (Are they being ageist!!??)
"Were you born in 1970 and will also be 40 in 2010? Enter for Free!
If you were born in 1970, bring your passport with you to the park between March 1st and 31st October 2010 and enter for free. Excludes special event days. Cannot be used in conjunction with any other offer including Tesco Clubcard vouchers."