So, my Astra A16 LET 1.6 Turbo recently sprung a leak as the thermostat housing had split.
Replaced the thermostat and housing and the 2 temperature sensors (one in the housing and 1 on the radiator), which fixed the leak. However, the fan is still constantly on maximum all the time, coolant level low message appearing (I think the radiator is empty as the reservoir is filled up and doesn’t drop), and the coolant temp won’t get over 75C using an OBD reader. 

No codes are present with the OB