Apparently the fence plank had just dropped, and staff were already on their way to fix it. Yes, the fence could have been maintained better, but it was one fence plank, easily missed.
The girl's parents should really have stopped her from running too far ahead and too far to the side tbh.
I'm sympathetic for the girl and her family, I really am, but I think 150K is far too much.
At the prices people pay to visit parks like Chessington, you expect to not be in any danger. Particularly at Chessington which is primarily aimed for children, you can't take any chances with H&S.
They should have noticed that that section of queueline was rotting, and taken appropriate preventative measures to minimise the risk of any incident occurring. This is something they failed to do and that young girl ended up with quite serious injuries because of it.
How can you say that the girls parents should have stopped her running ahead? They would have expected her to be safe anywhere in the park so you can't pin any blame on them.
The responsibility for the accident lays solely with Chessington/Merlin as they failed in ensuring all publicly accessible areas of the theme park were properly maintained.