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You may have also noticed that the engine temp would climb when stationary but drop when driving (which seems the opposite of what you’d expect)
Yep exactly this. Looks like its time to get on ebay and dig out the Haynes
Turkish-
Lifted off the web:
“Checking thermostat operation:
On some vehicle models, it’s easy to diagnose a thermostat.
Make sure the engine is cool.
Engage the parking brake.
Set the transmission to Park (automatic) or Neutral (manual).
Start and idle the engine.
Pop the hood open.
Remove the radiator cap.
Take a look though the radiator opening.Coolant should not flow at this point until the engine begins to reach operating temperature, usually in about 15 to 20 minutes.
If coolant is flowing with the engine cool, the thermostat is stuck open and needs to be replaced.”
Nef
That basically, not getting to temp, especially in the winter.
If the thermostat is stuck open, the coolant is always flowing through the radiator and being cooled. If the air temp is low and you’re driving quickly the radiator will be getting really cold, and thus the engine will be getting overcooled (hence your low temps on the motorway - extra airflow = extra cooling). You may have also noticed that the engine temp would climb when stationary but drop when driving (which seems the opposite of what you’d expect).
Your car should always get to temperature as it’s designed to, and the engine won’t like running cold.