kawasaki gpz600r 1987 thermostat coolant sender unit

[Busby]

I've had a problem with my temp gauge being erratic, moving from cold to mid temp then back to cold again, fan comes on for a short time then stops after switch off. Diagnosed that the temp gauge is actually working correctly. So the only thing it can be is the coolant sensor in the thermostat housing. GPZ zone replacement part is £19 + postage (aftermarket part), OEM part is over £30 + postage. As I live in Spain this is quite a lot! Looking at other options, the GPZ500 coolant sensor unit fits most of the early GPZ range. (same price though) I wonder why its so elffin expensive? Similar car coolant sensors cost about £8 to £10!