I was a in a traffic jam last night for about 1.5 hours just crawling along it was only 8 degrees outside and the engine was getting so hot that the red light next to the temp gauge came on, I turned on the heater full and the temp went down almoststraight away, but when I turned the heater down again the engine got hot to the same extent after a few minutes.On checking the water level it is full, does any one think it could be the coolant fan or the switch that controls the fan? How do I test it and where is it?? Please !!

 

I'd say fan or thermostat.

The easiest way to test without dealer level diagnostics is to get it to temp aand see if fan cones on.

No diesels were sold in the US so you might want to check the UK forum