My 2003 octavia elegance tdi 110bhp can be driven continually along the motorway at 70 and the temp needle barely moves off the scale. I put the blower on HI and get luke warm air being blown out. If I come to a stop and the car idles, the temp will slowly go up. Start driving again the temp shoots down again. Ive had a recent cambelt change and had the pump, thermostat and temp sensor changed but no difference. The problem still existed prior to the cambelt change but now its just getting worse. The outside temp is near 12c recently so shouldnt effect the warm up.

Anyone any ideas? I can only think that coolant is bypassing the thermostat somehow? I want to get it sorted as its affecting my mpg which is averaging 45mpg although the dash always says 55. Prior to the problem I always had 50mpg plus

That would sound like your thermostat is stuck open.... I know you said it was changed, but maybe your old waterpump was starting to break up and bits of it were jammed in the old stat, and some more have floated around and jammed the new one too....

At 12 deg C I would expect my car to be up to 90 deg C after about 6-7 minutes of driving (around town), so 30mph-ish, but my car is garaged overnight, certainly I would expect to see normal temp after 10 minutes - even on a cold cold night.

Have you felt the hose after the thermostat to see if it's getting hot coolant coming through?

It does sound to me like the thermostat is stuck open so the coolant is never getting hot enough (unless you stop somewhere at which point the radiator isn't cooling so effectively so the temperature will go up until the fans kick in).

Perhaps the new thermostat that was fitted is a duff one? But odd that it would do it before AND after all those bits were changed.

If it's running cool then that will definately affect your MPG.