Dave said:late opening is not financially viable.
I find this to be a European park trait in that they cut the hours because the last hour is always one that makes the least profit but then the parks give its guests no reason to stay till the end of the day unlike (dare I say it) Disney who throw on a fireworks show or major performance in the majority of their parks. I am not saying Merlin should go down that route due to costs, but just closing the ride gates at 6pm isn't a reason for anyone to stay.
The frustrating thing with Chessington is that it could do something like a parade around the park themed around its areas if it improved its infrastructure, reinvested in its own appearance and updated itself. It's criminal that Vampire hasn't seen a proper lick of paint in 22 years for example and the way Runaway Train is degrading is ridiculous. New rides are one thing, but actually keeping the upkeep of the rest of the park really should be a priority. 2016 is to far away, we're talking six years after the parks first proper new ride since Dragons Fury.
I think a dark ride is an excellent idea but the park needs to be inspired and take the initiative with it. Needs to lead not follow.