Thorpe's (or any park engineering team) I believe are on standard contracts so get a proper wage rather than an hourly one (I too looked into it a few years ago now)...
Average you're looking at for a mech eng is around £25-30k... I'd assume most of Thorpe's are at the lower end (or less depending on experience)...
Might be an idea to keep an eye on Blooloop for any theme park (or leisure industry) jobs going...
Benin's post in Thorpe Park Engineer Salary? was marked as the answer