When the light first came on I thought it was a temperature sensor not a header tank level indicator so I stopped and checked everything visually. The temperature guage was showing 90 degrees, as was the computer. Being new to the car I didn't open the header tank because I thought it was under pressure, especially since the screw thread on the cap is quite fine and takes a lot of turns to remove. I saw that water was flowing back into the header tank through the return pipe so I figured it was OK. I'll check it again tonight.