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 ECU gets its data from the standard Vauxhall sensor, which on these engines is part of the thermostat housing. Watching the temperature rise on the computer (I have an app called EasyMap which monitors all the variables in real time) you can see that the fan is triggered exactly on schedule at 100C. So I don't think there's an issue with the data that the ECU is getting, more that the coolant may not be circulating as effectively as it should.
Having done a bit of digging around on t'internet, it seems that it might be the thermostat itself which has failed, apparently there's a seal in there which can break up. If it's not opening correctly that would explain the higher than normal operating temperature.
So I've ordered a replacement thermostat along with a new switch sensor for the gauge. Not tremendously surprising I suppose, the Smiths sensor is 40 years old and the Thermostat nearly 20. We shall see.
