This was my solution, a contactless sensor stuck to the outside of the coolant tank. No floats to get stuck over time and its worked faultlessly since I put it on last summer.
It fired up about 2 months ago when my coolant dropped just below the halfway mark, which got me worrying a bit, but everything seems ok for now.
I think the sensors are around the forty to fifty quid mark, I managed to blag a free sample from the guy who supplies us with many of these type of sensors at work.
The brackets are Marklin, a type of German Meccano, to clip the sensor on to the expansion tank. A bit of Sikoflex then held it in place.
I utilised the unused automatic gearbox lamp on my instrument panel as a warning light. A small relay is used to invert the sensor output so it triggers when the sensor does not detect liquid.
Seems unaffected by the high temperatures on the expansion bottle too.
Total cost of the whole system is about £70
Sundance
Sensor
Fitted to tank
The inverting relay fitted under the binnacle
Shown working on the instrument panel, the amber light to the right