On the way back from Norway the car decided to chuck out all its coolant and peg the temp gauge at best part of 130ºC. After letting it cool and topping up the coolant everything was fine - but it was a particularly cold night coming back - so possibly just got away with it due to forward motion?
Today I let the car run at idle, part of its static monthly exercise, running up to full water temp and letting the oil circulate.... the coolant gauge again jumped up over max temp and on switch off blew the bulk of its coolant out of the overflow onto the garage floor.
Something then is still not right.
I wasn't really watching it so not sure if the radiator fan cut in when the engine was running.
Tests so far
Confirmed the ECU is seeing a temp reading - check
Used the ECU software to check the ECU could turn the radiator fan on - check
Swapped the fan relay with the aux panel one to confirm identical behaviour - check
Removed the thermostat and checked it opened around nominal 91ºC - check
Next up, monitor the ECU temp reading and see if its seeing the coolant temp rising and see if the fan relay keeps flipping - not sure if that is because the battery is low due to the car not running?
...
Update - sensor replaced issue fixed.