Update, warmed up with the thermocouples installed, taking readings every minute, then took the car for a run and got reasonable data.
The thermostat is an 80degC model and does open at 80degC according to the thermocouples. On the road from the temperature data around the radiator and pump there seems to be no issue with temperature control, because on the open road, the thermostat temperature maintained between 78 and 82degC. From this new data the temperatures are as expected, and not cool as the dash gauge suggests.
At Idle with fans on, the temperature is controlled by the Davies Craig controller, which switches fans on at 88degC.
What I cannot understand is the data from the engine side.
At idle with fans on, the thermocouple at the Engine OUT position, the Dash reading from the engine block sensor, and the DC controller read the same as the Tstat IN, which seems to prove that calibration is good.
However, on the road, while the Tstat IN temperature is controlled to 78 to 82degC, the Engine OUT and DC readings drop to 75degC at 40mph and 72degC at 70mph, while the Dash reading (engine block) drops to 70degC at 40mph and 65degC at 70mph.
To have an engine temperature which is lower than the Rad IN temperature is impossible, so I don't know why it appears to drop with speed. If it was a measurement issue, why would they read the same at ambient temperature, and at 88degC, but have a significant error in between?
Still thinking about it.
Randy, I had already tried radiator blanking (1/3 of rad area), but it just compromised the cooling at idle.
Dave