Well I pulled the dash apart and took out the coolant gauge. There's not much to it. A circuit board with one resistor and a coil of very fine wires inside. The tacho has to come out before you can extract the gauge because it's dial overlaps the coolant gauge.


As I said before with the ignition on the gauge was reading 175ºF with or without the sensor connected and I also found that with the 4-pin connector to the dash removed which carries the ground to the dash, the sensor connection, the charge warning light connection and the dash illumination that the needle moved up to around 110ºF with the ignition on, so it was finding a path to ground through something.
I also realized that since the scale starts at 105º there was no way I could set the needle properly as there is no zero position. If it started at 60ºF it would have been easy as that was about the ambient temperature yesterday and the sensor had a resistance of about 7KΩ
With 12v connected to the +ve terminal and ground to the negative terminal the needle, as expected, rose to 175º. Without some resistance representing the thermistor at running temperature of around 160ºF the best I could do was pull off the needle and put it back at the bottom of the scale at 105º.
While the dash was off I took the opportunity to clean all the flexible circuit connectors. What a stupid arrangement to use on a motorcycle where the elements can get it. The only connector to show signs of corrosion was that 4 pin one next to the coolant gauge.
So put the dash back together, connected everything up and turned on the ignition. Bugger! I had expected the gauge to stay at the bottom of the scale, it now reads about 140ºF. So an improvement in the right direction but still nowhere near right. Once I get some petrol in the bike I can get it warmed up and see what the new normal is.