Hi James
What do you mean by an "end stop"?
If the gun won't cock check the length and coil number of the spring. It may be coil bound?
If it worked before and has just stopped working then strip the gun and give it a clean and make sure the safety catch mechanism is comletely free from any dirt and put a drop of light oil on the spring and on the safety catch itself before pushing it back into the back block.
Cheers
Opportunity is missed by most people, because it is dressed in overalls and looks like work.