The "low coolant" warning light started coming on, at first intermittently, but then just stayed on even though the tank was full. The level sensor had failed, quite a common issue across the Range Rover/ Discovery models.
It seemed a fairly straight forward task to replace the whole tank with sensor as it's pretty much the same cost as just the sensor.
It turned out to be an easy task except for the lower hose and clamp being rather fiddly. I believe access is a little easier on some models depending on the engine.

Here's a little video if you're interested.

Are you sure it was a faulty sensor and not just a weak antifreeze mixture?

It's a very common fault, the float in the sensor becomes porous (from a little reading on the subject) and gives false "low coolant" warning along with the dreaded Red triangle of doom.

djb222 said:

It's a very common fault, the float in the sensor becomes porous (from a little reading on the subject) and gives false "low coolant" warning along with the dreaded Red triangle of doom.

Click to expand...

A mate of mine who is an Indy RR parts supplier advises to change the whole expansion tank.