On cars designed by rational people, there is a reserve or overflow tank made of translucent plastic so you can see the level, and there are marks for “Low” and “Full” – full being nowhere near the top of the container, it’s usually about halfway up. On some cars these reserve tanks are pressurized, on others they are not, either way works.
Jaguar devised just such a reserve tank, calls it a “header tank”, and made the @#$%^ thing out of metal so you can’t see the level without opening it. And, understandably, owners conclude it’s supposed to be full right up to the cap, and are constantly popping the cap and adding – probably contributing to the huge loads of minerals plugging up the radiators.
Jaguar eventually realized that scheme wasn’t working so they added an “atmospheric catchment tank” – basically a reservoir for the reservoir. They made it out of translucent plastic – then hid it within an enclosed compartment on the XJ-S so you couldn’t see the level anyway! And then connected it with two different size hoses with an adapter that gets plugged up and causes blown radiator hoses. And they hid that adapter where you can’t easily find it, either. Of course, owners still pop that radiator cap on the header tank to check the level, which causes the coolant to drain back into the atmospheric catchment tank. Then they top it up. Repeating this a few times results in the atmospheric catchment tank overfilling and dumping coolant within the bodywork, where it leaks out of everywhere behind the LF wheel. One could reasonably conclude that Jaguar was deliberately out to cause problems with this system.
There are lots of proposed fixes, but none of them involve leaving the system in OEM configuration! In an ideal world, someone would make or find a translucent plastic container to replace the metal header tank, complete with reasonable “Low” and “Full” markings, and the atmospheric catchment tank would be jettisoned, a simple overboard hose installed instead leading out the bottom of the engine compartment. Failing that, you could fairly simply modify the OEM metal header tank by brazing in a fitting near the bottom and another just below the radiator cap and connecting these two with a length of clear tubing. Hence, you could easily see the level within without uncapping it.
Others have suggested replacing the hose between the header tank and the atmospheric catchment tank with a clear hose so you can watch the coolant move back and forth in it. Of course, ya gotta do something about that plug-prone hose adapter in the line, preferably tossing it over the hedge and running the larger size hose the whole way.