Land Rover on most of the later vehicles use a plastic screw in the hose system to bleed the air out of the radiator system. These plastic screws seem to go brittle over time and can snap when bleeding or when driving. If it snaps when you are attempting to bleed the system the slug "threaded section" has to be removed. In many cases on attempting to remove the slug causes the plastic insert to fall into the radiator and a new hose is required. If it fails while driving the coolant is pumped from the engine and it is possible to do engine damage. The solution to this problem is to fit a brass replacement plug before it becomes a problem. If your car is still plastic “Change it!”