Last night I started driving back from Glasgow to Edinburgh and as I set off I noticed the temp gauge was much higher than usual so I stopped and looked under the bonnet. There was no steam or anything, so I waited 5 minutes then set off again. It stayed normal til I got onto the motorway when I started accelerating past 50mph it started going up really fast again. I slowed down to about 45 and it went back down, and I kept going at that speed for a while. It stayed at normal, so I went up to 50mph and it stayed fine there too. Near the end of the journey I tried going up gradually to 65mph and it still stayed normal.
Any idea what was causing it? It's been ok today when I went to the supermarket.