Water/Oil Temp Gauge

Did it work OK previously? Not all senders work with all gauges, they must be matched. If the gauge or senders have been changed at some point in the past, they may not match. Obviously if it used to work fine with the same components, this won't be relevant.

OK if you have a multimeter you can measure the resistance of the water temperature sender when the engine is properly warmed up. Just measure the resistance between the terminal and the brass body with it in positon (be sure to unplug the connector first or the resistance of the gauge in parallel will throw it out). The measurements below are taken from a standard Caterham sender (they're not officially published calibration data, just measurements I took from mine which reads normally with a standard Caterham gauge when I was trying to look around for cheaper alternative senders with the same calibration - I failed by the way!). As you can see (from extrapolating back a bit) the correct resistance at around 80°C is so completely different to that at 40°C that it should be pretty obvious whether it's the sender that's that far out or whether it's the gauge (I would say that such a big difference in readings is unlikely to be down to resistance in the wiring connections or switch). The senders are very well known for failing and when they do they go high-resistance (under reading, although usually intermittently or with the gauge flickering up and down).

50 °C 624 Ω
55 °C 520 Ω
60 °C 422 Ω
65 °C 351 Ω
70 °C 283 Ω
72 °C 262 Ω
74 °C 243 Ω
76 °C 228 Ω
78 °C 211 Ω
80 °C 191 Ω
82 °C 179 Ω
84 °C 164 Ω
86 °C 151 Ω
88 °C 140 Ω
90 °C 128 Ω
92 °C 119 Ω
94 °C 109 Ω
96 °C 102 Ω
98 °C 93 Ω
100 °C 85 Ω

Otherwise I have a spare genuine Caterham gauge on my spare engine test rig that you are welcome to borrow if you want to see if it cures the problem. BM me your address if you want it and I'll put it in the post.

9 times out of 10 it's the sender.

I would agree. There is very little to go wrong inside the gauge itself. A coil of wire around a magnet. If the coil breaks, it just does nothing. The only thing likely is the machanism sticking but this would be obvious (it would warm up to 40°C in normal time then just stop).

My pleasure, let us know how you get on :)