Mr G, the fuel level sensor in my Evo is a thermistor type and, when I had the pump assembly out to change the filter, I had a little play around with it. (I know, I should get out more!)
As already stated, the thermistor does operate on a temperature basis. The way it works is that it draws a small current from the bike, which heats it up to a certain temperature. When it's immersed in petrol the fuel acts as a heat sink and keeps the thermistor at a constant, lower, temperature regardless of the ambient temperature. At the lower temperature, the resistance is high but, when the fuel level drops below the thermistor, it heats up, which reduces the resistance causing the fuel light to come on.
Be aware if you're testing it, that it may take 30 seconds or more for the thermistor to heat up enough to switch the light on.
Hope that helps.