Radiator Fan and Warning Light

[Doily]

The fan and water temp warning light is coming on when, according to the gauge on the dash, the water is not hot enough (the needle on the gauge is just over half way). This has never happened before in the 10 years I've had the car.
I have a 2003, 1.1, TU1JP engine. No a/c and no radiator fan switch screwed into the rad.
Eventually, after a lot of fan running time, the needle drops to one quarter up the scale and the fan switches off.
Has anyone got any ideas please?
Also, as I may have to play parts darts to fix it, can anyone please tell me what the two sensors do, that are near the thermostat housing : A green one, screwed into the top of the thermostat housing with two wires, green and white; and a blue sensor, screwed into the end of the cylinder head with only one wire, which is green (I took some pics but couldn't figure out how to put them on this message!?).
TIA.