I've had the car for 8 years, and recently the "Engine coolant low" alarm appears. I have removed the expansion box" screw cover, but I find that there is another object that prohibits me from topping the reservoir up. This object is black plastic with a brass insert that is nestled within the neck of the expansion chamber.Does anyone have any suggestions on how to top up the coolant on this engine please?. I know it sounds very basic, but it is so frustrating not to be able to complete such a simple task and I am unable to find anything in the user manual