Somebody had the same problem some time ago and this was my
reply. Never heard/got any reply so may of helped or
possibly another piece of useless info???

I had that problem with my 306 Lx D. As others have said it
is serious WHEN it comes on with another indication (water
temp or oil pressure) in my case there were no other
problems or indications. It turned out to be a corroded
connector on the water temp sensor. There are  two wires to
the sensor one is for the temp sensor and the other one is
an over temp switch to earth which will bring on the STOP
light, this was corroded and was some how going to earth and
intermittenly putting the light on. The pin and the
connector corroded so bad that the only cure would be a new
sensor and connector. A new sensor is not a problem but
the connector (blue IIRC) is only available in the form a
new wiring harness at some horrendous price, needless to say
it has not been replaced, a visit to the scap yard for a
connector will suffice when convenient. The water temp gauge
has been working  Ok for last few months and the STOP light
has not come on when it shouldn't.