stop light flashing in citreon xsara (2001)?

Had a bit of a scare this weekend when the red warning light over the temperature gauge and the 'STOP' signal on the dashboard started flashing at me. The thermometer shows nothing out of the ordinary - 90 degrees or so - but I've topped up the oil and water. It seemed to help for a while but then the same thing started happening again, but intermittently.

Can anyone please help or advise.