Last night I drove from Glasgow to Gloucester, When I first started the car and drove the gauge went to just about the normal working temp after a few minutes, but when I started to drive on the motorway the temp gauge started to drop and would not give a reading, I pulled over at the services and after a few minutes the temperture gauge started to give a reading but not as high as the normal working temp and I checked the hoses which where warm, I started to drive again and the gauge dropped again, I turned the heating on to full it was warm but it wasn't as warm as I would expect it to be (picture below)
Any Ideas??? Thermostat stuck open??