I have a 2.0 tdci estate. I live in central europe. Now in the summer I have to watch the temp gauge going up hills on the motorway. The temp gauge usually sits halfway on the flat and up hills.

But if the outside temp is 30+, and I'm doing 80mph or a bit more (it's legal here) and I'm going up a hill, the gauge almost hits the red. When I slow down to about 60, it goes back to halfway.

Winter, when its -20, still sits at halfway going up a hill and lower on the flat and going down.

Local mechanic says the thermostat is ok. Water pump, I don't know