Temp gauge and heater oddity

[pandp38]

On trip to lake district, after 140 miles the water temp gauge pointer went from normal 12 o'clock position nearly all way over to the left in the space of 5 miles. I checked the heater and it was blowing cool when set on hot and fan on no.3. After another 20 miles or so the temp started rising again and heater was blowing warm.

As it was raining and we were fully packed for camping I didn't look in engine bay at the time but at campsite all looked ok.

On return trip home,150 miles all was ok.

Any ideas? Thermostat, air lock, water pump ?

Paul
1989 1.9DG WBX