The temp sensor that's been installed in the return pipe is as you quite rightly say of the type normally found in the block or lower third of the radiator and sounds like the coil may be breaking down. Better here to change the whole housing and sensor out for a specifically designed in line (pipe mounted ) sensor. Where does the ECU get its control signal from? This would be my first point of investigation, you have checked flow on the stat opening and presumably it closes again as the temperature drops. The fan came on when you didn't think it should have and indeed hasn't previously. If you want to get better control of the temperature the fan switches in at then perhaps an electronically controlled in line sensor switch might be a good idea, The whole kit including T piece, sensor and control unit are less than £70. Once you have found the problem then this might be a way for you to fine tune when the fan kicks in, in other words, fine tune your unorthodox car for best function. I think you have to view your car as you would a kit car, Clearly the variables here are from a non designed set up, It may have worked alright when all the parameters of everything in the system were managing to overlap and the simplest remedy would be to find the failing area/part and replace it or perhaps look at the cooling system as a whole.
