Hi Steve,
The central computer of the Xsara and Picasso can be reset by disconnecting the battery for about 10 minutes. I had to do this with our Xsara after changing an O2 sensor to remove a fault light. The car has run perfectly since this.
I would only do this if you are sure there hans't been a fault in the engine and the light has just come on for no apparent reason. If it comes back, go to a Cit workshop or someone with an OBD reader and check the fault code!