I had an issue on the way back from the IOM with my Sigma 150, with the help of some fellow 7'ers it was decided that the relay had worked it's way loose or had muck on the contacts on the terminals - once it was removed and confirmed to be working in another 7, all was well again.
I'd quite like a fan mod, so I have more control, but as Jason and SKC point out, the sensor is the ECU, so not sure of the best route to "do" the mod (other than fused wire from battery to a switch and then off to the fan - which seems altogether too simple ❗)
Mike