This is an odd one. I found plenty of help on the forum for this problem but not in the X350 threads.
After first seeing the Coolant level low message I have done the following.,
Filled up a bit with fluid - this helped for about 2 days but the message and the red light re-appeared.
Installed a new sensor - didn't help.
Installed a new tank and another new sensor - has still not cured the problem.
I now have 3 sensors, 2 tanks and a very insistent error message.
So am now left thinking the wiring must be where the fault lies.The 2 cables connected to the sensor seem to join other cables about 6 inches lower down and then disappear under the radiator. I really don't want to have to remove that to see where the cable goes so now my question. Does this small cable harness go onto a connector somewhere under the radiator? Or where can I find test points to check the 2 cables going to the coolant level sensor.
Anybody know about the wiring on the X350?

Some coolant level sensors will trip an error when the coolant is down just the smallest of amounts from full. On my 04 XJ, my first tank would trip an error if the level dropped more than about 1/4" from full. My present tank/sensor trips if its about a 1/2" drop before tripping the error which still isn't a whole lot of coolant loss.

Before throwing more parts at the problem, you might want to bench test the tank sensor with a multimeter. Since adding fluid cleared the error for a couple days with a prior tank, I think you might need to consider that the error is telling you that you do have a small coolant leak. The few leaks I've had in my X350 have been small - or at least I was able to resolve them while they remained small.

Thanks rhankey for the reply. This error has been showing since about 6 months now in total. It was only the first time that a little more coolant helped. Since then I have been watching the level very carefully and there is definitely no leak as the level remains above minimum all the time.
I have now also checked the original tank with the original sensor. There is no continuity when the level is too low. As soon as I have enough fluid in the tank then my multi-meter shows continuity. So the sensor is working correctly when the tank is suitably filled.
Is there a fuse for this sensor? I can see nothing in the book but you never know.
Otherwise i am forced back to the conclusion that it is a wiring issue.
But where does the little wiring harness for the sensor connected up to a bigger harness?
Any ideas?

It sounds like you have a small coolant leak somewhere. In fact, the coolant level sensor inside the coolant reservoir is performing as it should so this would hardly be an electrical problem.

The tank and sensor have been tested today out of the car in my bathroom sink and are working correctly.
The error message is immediately on when I start the motor with a full tank of coolant and never goes off irrespective of how much coolant is in the tank - even filled to the brim!. There is definitely no leak. Earlier this evening as a next step I shorted the 2 contacts in the connector which theoretically must switch the error off. It did not. I am left with no alternative but to question the wiring. I am not sure though where this cable goes - anybody know?
I also very stupidly dropped the lower water hose clip when removing the tank.Is there a replacement available - I couldn't face having to buy a new water hose just for a 2 cent clip.

Maybe rebooting the car will solve the problem?

Have you removed the undertrays?

If you can't obtain a clip you might consider making something

with wire and maybe a cable tie.

Look at the plastic coolant rail between the engine and the radiator. there is a nipple for the small bore pipe that goes to the back of the header tank. On my 2003 XJ6, this was close to breaking off. In fact when I got to the independent Jaguar workshop I was using, they told me to leave the car with them as the pipe could break away any time and cause a total loss of coolant very suddenly with the possibility of wrecking the engine. The nipple on the header tank that takes this small pipe also cracks and eventually falls off, but usually the loss of coolant is noticed. In my case, I had to have a new coolant rail and header tank. Not cheap, but cheaper than a new engine !

That information wasn't overly clear in your initial message. Sounds like it is now time to check electrical continuity from the sensor wires, through the electrical connector and then at the ECU. I would expect corrosion at one of the connectors. Not having looked at the wiring diagram, I would expect one the two wires goes to ground. If so, the first place I would check is your ground points which are known to corrode on our Al cars.